CLASSIFICATION AND PHYLOGENY OF THE SUPERFAMILY
PLATANISTOIDEA, WITH NOTES ON EVIDENCE OF THE
MONOPHYLY OF THE CETACEAZHOU KAIYA
Department of Biology, Nanjing Normal College, Nanjing

ABSTRACT

Important basis provided by the morphological studies on the skeleton,
digestive and respiratory organs of the Platanistoidea have further provedthat this group can be divided into four families. Considering the characteristics of the skeleton and other morphological features, the phylogenetic
relationships among the four families are discussed. The systematic sequenceof the families should be Iniidae, Lipotidae, Pontoporiidae and Platanistidae.
Some characters noticed in the investigations are evidence in favour of themonophyly of the cetacea.
INTRODUCTION

The superfamily Platanistoidea possesses a number of primitive characters similar
to the Oligocene-Miocene Squalodontoidea, some characters similar to the primitive forms of the higher cetacean families and some specialized structures. Two
different opinions advocating divide or combination concerning the classificationof the Platanistoidea have been held over a long period of time.
In the middle of the 19th century, Gray (1863, 1866) first divided the Platanistoid dolphins into groups. In his catalogue, Platanista constitutes the fourthfamily of Cetacea-Platanistidae; Inia constitutes the fifth family-Iniidae; Pontoporia was placed into the sixth family-Delphinidae. Flower (1869) gave another
opinion before long, put together the above mentioned three genera into thePlatanistidae. He made Platanista and Inia belong to the subfamily Platanistinae
and Iniinae respectively and placed Pontoporia into the subfamily Iniinae provisionally.
At the beginning of the 20th century, Miller (1918) named Lipotes, the fourth
genus of the modern Platanistoids, and referred it to the Iniidae. In anotherpaper published in 1923, he followed Gray in recognizing Platanistidae and Iniidae
and still considering Pontoporia a member of the Delphinidae. Nevertheless, hehas placed Pontoporia into the subfamily Stenodelphininae to differ from othergroups of the Delphinidae. Kellogg (1928) held the same opinion as Miller did,whereas Winge (1918) maintained to put Platanista, Inia, Pontoporia and the newly
discovered Lipotes together into the Platanistidae. In this period, argument concerning divide or combination of the Platanistoids was continued and different
opinion was set in whether Pontoporia belongs to Platanistoids.
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In 1936 Slijper adopted the concept in favour of combination and placed all
the living genera of Platanistoids into the family Platanistidae. Following Slijper's
opinion, Simpson (1945) recognized Platanistidae but retained three subfamilies
and, moreover, established superfamily Platanistoidea in the higher category.
This taxonomical system has been accepted by most cetologists ever since. Slijper's
system has been adopted by Norman and Fraser (1948), Slijper (1962), Nishiwaki(1965, 1972), Tomilin (1974), Matthews (1978), Lockley (1979), and others.
Simpson's system has been followed by Fraser and Purves (1960), Carvalho (1961),
Marcuzzi and Pilleri (1971), Brownell and Herald (1972), Mead (1975), Rice (1977),
and others. The single family system was also adopted by Hershkovitz (1966), but
a new name Susuidae which he proposed has not been accepted by other cetologists. Authors studying fossil Platanistoids such as Kellogg (1944, 1955 ), Rensberger(1969) have continued to recognize the iniid as a distinct family. Gaskin (1976)
who is studying modern cetaceans has followed their opinion.
The investigations of Platanistoid dolphins have gained great attention again
since 1960s. Kasuya (1973) recognized the Platanistidae, lniidae and Pontoporiidae of the Platanistoidea on the basis of the study of the tympano-periotic bone.
In a paper on the study of Lipotes, van Bree and Purves (1975) put it in the lniidae.
Pilleri et al. (1976) proposed to separate Inia, Lipotes and Pontoporia from the
Platanistidae and put them in the lniidae. The coexistence of the one familysystem (Slijper, 1936; Simpson, 1945) with the two and three family systems(Pilleri et al., 1976; Kasuya, 1973) occured. In 1978, Zhou et al. proved that
Lipotes is not closely related to Inia and the differences between the two genera
exceed those between the Delphinidae and Phocoenidae, and placed Lipotes into
a separate family-Lipotidae. After this, Pilleri and Gihr (1980) have turned
to use a four family system, but no argument has been given.
The present paper summarizes the important basis provided by the morphological studies of the skeleton, digestive and respiratory organs of Inia (Flower,
1869; Mead, 1975; Lonnberg, 1928; Pilleri and Gihr, 1976c, 1977; Yamasaki andKamiya, 1981; Zhou, Li and Pilleri, 1982), Lipotes (Miller, 1918; Chen and Chen,
1975; Zhou et al., 1978, 1979a, b; Chen et al., 1980; Liu and Lin, 1980; Zhou andLi, 1981), Pontoporia (Burmeister, 1867; Flower, 1869; Carvalho, 1961;"Schenkkan,
1972; Yamasaki et al., 1974, 1975, 1977; Mead, 1975; Pilleri and Gihr, 1976b;
Yamasaki and Satomi, 1976) and Platanista (Eschricht, 1852; Anderson, 1879;
Arvy and Pilleri, 1970; Yamasaki and Takahashi, 1971 ; Takahashi and Yamasaki,
1972; Yamasaki et al., 1972; Purves and Pilleri, 1973; Pilleri and Gihr, 1976a;
Yamasaki, Komatsu and Kamiya, 1977) and further proves that the Platanistoids
can be divided into four families. The phylogenetic relationships among the fourfamilies and the evidence in favour of the monophyly of the cetacea are also discussed.
BASIS OF THE CLASSIFICATION OF THE SUPERFAMILY

An uniform understanding of the genera of Platanistoids has been acquired earlySci. Rep. Whales Res. Inst.,
No. 34, 1982

CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEATABLE I.

IniidaeLipotidae
Pontoporiidae
Platanistidae
Ziphiidae
Physeteridae
Monodontidae
Delphinidae
Phocoenidae
Balaenopteridae

95

COMPARISON OF THE COMPARTMENTS OF
THE STOMACH OF CETACEA
Fore-stomach

Main stomach

Connecting
channel

0
0

3

0

0

Pyloric stomach

1
2
0
0

many compartments

1

0

in the third decade of the twentieth century. However, no identical result on
the classification in family rank has been obtained so far. The materials studied
which were mostly confined to skulls have something to do with the matter. When
studying the affinity between Lipotes and Inia, Zhou et al. (1978, 1979a) comparedattentively the vertebrae, sternums, flipper skeletons and other available materials
of external and internal structures in addition to the careful comparison of thestructure of the skulls. As a result of these studies, the close relationship between
Lipotes and Inia was refuted. In recent decade, the studies of the skeleton anddigestive tract of Platanistoids by Yamasaki et al., Pilleri et al. and Zhou et al.
have provided important basis for the classification of the modern Platanistoids.
Table 1 indicates that the stomachs of other modern cetaceans consist offore-stomach, main stomach and pyloric stomach except those of the Ziphiidae
in which the fore-stomach is absent. They differ from each other only in thepresent (Delphinidae, Monodontidae, Phocoenidae) or absent (Physeteridae,
Balaenopteridae) of the connecting channel. The stomachs of different groupsof Platanistoids differ from each other not only in the above mentioned characters,
but also in the subdividing of the main stomach or not (Fig. 1). Both Inia andPlatanista possess fore-stomach, but the main stomach is single chambered in theformer and divides into two compartmei:its in the latter. In Lipotes and Pontoporia
the fore-stomach is lacking. The main stomach divides into three compartments
and the connecting channel is absent in the former, while the latter possesses single
chambered main stomach and the connecting channel. The differences between
the stomach of any two groups of Platanistoids go beyond the mutual differences
between the stomach of most of the odontocetes. Refer to the Artiodactyla, thenumber of compartments of the stomach goes so far as to be the main character indistinguishing Infraorder Tragulina (3 compartments) and Infraorder Pecora (4compartments). Thus the structure of the stomach is one of the key charactersin weighing the degree of separation among different groups of Platanistoids.
The structure of the skeleton is still important basis in classifying the Platanistoids. Attention should be paid first of all to four note-worthy differences between
Inia and three other groups of Platanistoids. 1. The palatal portion of the two
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96

ZHOU

c

Fig. 1. Diagram of the stomach of (A) Inia geo.ffrensis, (B) Lipotes vexillifer, (C)Pontoporia blainvillei, (D) Platanista gangetica.
CC. connecting channel; DA. duodenal ampulla; DP. duodenum proper;E. esophagus; FS. fore-stomach; MS. main stomach; MSI. first compartment of
main stomach; 2. second compartment of main stomach; 3. third compartmentof main stomach; PS. pyloric stomach; HPO. opening ofhepato-pancreatic duct.

->Fig. 2. Ventral aspect of skull of (A) Inia geoffrensis, (B) Lipotes vexillifer, (C) Pontoporia blainvillei, (D) Platanista gangetica.
Bo. basioccipital; Fr. frontal; Mc. maxillary crest; Mx. maxilla; Pl. palatine;
Pt. pterygoid; Pth. pterygoid hamulus; Ptp. lateral plate of pterygoid; Sq. squamosal; Vo. vomer; Zy. zygomatic process ofsquamosal.

Sci. Rep. Whales Res. Inst.,
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CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEA

Sci. Rep. Whales Res. Inst.,.No. 34, 1982

97

ZHOU

98

ertob>-~· IniaDgeojfrenszs,. (B)f: L ;hotes. . (C) Ponvexillifer,ts (bold !me .

toporza
. vD)
of "
(A)
blaUwil••'( PMtam<ta
F"ig. 3. . Thorac1:" ' ". ' "howffig =<'1

•~

i,,

•

)

11,,.
"'·
&i. R.p. ff(Im/""34' Im<..
1982

CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEA

99

maxillas and palatines of lnia is separated by the vomer. In three other groups of
Platanistoid, the palatal portion of maxilla contacts that of the opposite side
(Fig. 2). 2. Only irregular tubercles are found on the ventral aspect of the pterygoid hamuli of lnia, whereas the pterygoid hamuli of Lipotes possess a thin plate
which recurves dorsally. This portion enlarges to form the lateral plate of pterygoid in Pontoporia and Platanista. 3. In lnia, the costal facets are located at the

A

c

B

D

Fig. 4. Flipper skeleton (not including phalanges) of (A) Inia geoffrensis, (B) Lipotes
vexillifer, (C) Pontoporia blainvillei, (D) Platanista gangetica. Ulnare hatched;pisiform stippled.
Sci. Rep. Whales Res. Inst.,
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ZHOU

100

anterior and posterior edges on each centrum of the first two thoracic vertebrae
and occured at the anterior edge only in the hinder thoracic vertebrae. In Lipotes,
they are located both at the anterior and posterior edges of the centrum of the firstvertebra and occured at the posterior edge only on the centrum of the 2nd-5th
thoracic vertebrae. In the 7th thoracic vertebra, the facet occurs on the anterior edge only. The costal facets are located at the hind edge on the centrum of
the thoracic vertebrae in most individuals of Pontoporia. In few individuals, thoseof 5th or 4th-5th thoracic vertebrae are situated at the front edge of the centrum.
All costal facets of the thoracic vertebrae of Platanista are situated at the hind edgeof the centrum (Fig. 3 ). 4. According to the radiograph published by Pilleri andGihr (1976a, b, c), the ulnare and pisiform are free in Inia; the ulnare is free but
the pisiform is missing in Pontoporia; both bones are missing in Platanista. According
to our specimens, although the ulnare and pisiform of Lipotes are fused to the ulna
and 5th metacarpal respectively, they are still distinguishable (Fig. 4).
The tympano-periotic of the cetacea has moved ventrally and lost the direct

A

c

B

D

Fig. 5. Connection between tympano-periotic and skull in (A) lnia geoffrensis,(B) Lipotes vexillifer, (C) Pontoporia blainvillei, (D) P!atanista gangetica, Waved edge
indicates suture of bones and dotted are the laminated structure. (A) and (C):
fixed with ligament; (B) and (D): direct joining. (Modified from Kasuya, 1973,)
Sci. Rep. Whales Res. Inst.,

No. 34, 1982

:"'·

~[

""'

~

1;;"

""""'

~~

......

""~
u""'~

~

~~

Costal facet ofthoracic vertebrae
Carpals

Palatal portion of
two maxillas
Bony plate of pterygoid hamulus
Connection between
tympano-periotic
and skull
Teeth
thin bony plate recurves
dorsally
posterior process of tympanic
bulla loosely sutured tosquamosalcrown with reticulate enamelrugositysmall tubercles on ventral
aspect of pterygoid hamulus
fixed with ligament

located mostly at anterior
located mostly at posterioredge of centrum
edge of centrum
7, ulnare and pisiform present 4--5, ulnare fused to ulna
and pisiform fused mostly to
5th metacarpal

crown with nodular enamelrugosityin contact

separated by vomer

absentnot reaching supraorbital
process of frontal

absentnot reaching supraorbital
process of frontal

located at posterior edge ofcentrum in most individuals
6, ulnare present, pisiform
missingcrown simple

enlarges to form lateralplate of pterygoid
fixed with ligamentin contact

absentreaching supraorbital processof frontal

enlarges to form lateralplate of pterygoid
posterior process of tyropanic bulla loosely sutured
to squamosalcrown simple, upper toothrows merged together andalmost in contact
located at posterior edge ofcentrum
5-6, ulnare and pisiform
missingvery well developed
reaching orbital plate offrontal from which a supraorbital process is usuallydeveloped
in contact

present
3.6-4.9

absent24.0-37.3

absent12.5-14.9

absent19-20

located at duodenum proper

Opening of hepatopancreatic duct
CaecumRatio of intestinal
length to body length
Maxillary crest
Zygomatic process
of squamosal

Platanistidae

fore-stomach absent, threefore-stomach absent, single
compartments of main stomach main stomach, connecting
and pyloric stomach present
channel and pyloric stomach
present
located at duodenal ampulla
located at duodenum proper

absenttransverse, crescentic
arising from trachea justabove bifurcation of rightand left bronchi

Pontoporiidae

fore-stomach, single main
stomach, connecting channel
and pyloric stomach present

Stomachabsentlongitudinal, elliptic
arising from trachea markedly
higher than bifurcation ofright and left bronchi

Lipotidae

absentlongitudinal, slitlike
arising from trachea
markedly higher than bifurcation of right and left
bronchifore-stomach, two compartments of main stomach, connecting channel and pyloric
stomach present
located at duodenum proper

sparsely on snout
transverse, crescentic
arising from trachea slightly
higher than bifurcation ofright and left bronchi

Iniidae

COMPARISON OF THE MAIN CHARACTERS OF THE FAMILIES OF THE SUPERFAMILY PLATANISTOIDEA

Hairs
Blow hole
Apical bronchus

Characters

TABLE 2.

0

--

;i:..

l:'l

tJ

0.....

i-,lC/J.

z.....~

r

"'C

><:
0>zj

~

Q

r

::c:
><:
0

"'C

tJ

~

z

~
.....
0

0

.....
>zj
.....

C/J.
C/J.

;i:..

r

0

ZHOU

102

joining to the skull in various degrees in the course of the adaptations to the aquatic
life. In Lipotes and Platanista the posterior process of tympanic bulla is still loosely
sutured to the skull (Kasuya, 1973; Zhou et al., 1979a). The tympano-perioticof Inia and Pontoporia is fixed with ligament to the skull (Fig. 5 ). Besides, Inia and
Lipotes differ from Pontoporia and Platanista in having the zygomatic process ofsquamosal not reaching the supraorbital process of frontal. The distinction between the teeth of different Platanistoids is as follows: The crown of the teeth ofInia and Lipotes is covered with nodular and reticulate enamel rugosity respectively
and that of Pontoporia and Platanista is simple. The upper tooth rows of Platanista
merge together and are almost in contact with each other. Platanista also differsfrom three other Platanistoids and all other living odontocetes in having the curious maxillary crest.
The morphological distinctions between different Platanistoids showing in
Table 2 are far more than those between different families of most mammals.
This fact proves that each of the four Platanistoid dolphins represents a separate
lineage derived from the primitive ancestor at an early stage of the evolutionary
process. Their taxonomic categories should be referred to family rank.
PHYLOGENY

The traditional view concerning the relationship of different Platanistoid dolphinsis that Lipotes is closely related to Inia and that Pontoporia is close to them. The
systematic sequence of these groups has been arranged as A and B by most authors(Simpson, 1945; Carvalho, 1961; Marcuzzi and Pilleri, 1971; Kasuya, 1973;
Tomilin, 1974; Mead, 1975; Matthews, 1978 other). It has been arranged as C
by Slijper (1936, 1962) and some authors (Norman and Fraser, 1948; Nishiwaki,
1965, 1972) according to the sequence of naming. There are still few othersequences (Fraser and Purves, 1960; Hershkovitz, 1966; Rice, 1977). According to
the informations now understood, these arrangements can not express the natural
relationships of Platanistoids.
Asubfamily Platanistinae(or family Platanistidae)
genus Platanista
subfamily Iniinae(or family lniidae)
genus Inia
genus Lipotes
subfamily Pontoporiinae(or family Pontoporiidae)
genus PontoporiaBcfamily Platanistidae
genus Platanista
genus Inia
genus Lipotes
genus Pontoporia

family Platanistidae
genus Platanista
genus Inia
genus Pontoporia
genus Lipotes

By inference from the analysis of the characters, the phylogenetic relationships
among the families of Platanistoids may be expressed as Fig. 6. The Iniidae is
a side branch diverged from primitive Platanistoids. This family is characterized
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEA
Pontoporiidae
IniidaeLipotidae

Platanistidae

Hairs on snout
Palatal portion of maxillas
Bony plate of pterygoid
Costa! facet of thoracic
vertebrae
Ulnare and pisiform
Zygomatic process of squamosal
Crown of teeth
Opening of hepato-pancreatic duct
CaecumCo1,1n~ction between tympanopenot1c and skull·

Maxillary crest
Upper tooth rows
Apical bronchus

0

Plesiomorphous character•

Apomorphous character
Symplesiomorphous character
Synapomorphous character

Fig. 6.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

Diagram of the phylogeny of the superfamily Platanistoidea,

103

104

ZHOU

by apomorphous characters, i.e. the separation of the palatines by the vomer andthe location of most of the costal facets at the anterior edge of the centrum of thethoracic vertebrae, which differentiate lniidae from other Platanistoids. The position of the head of the ribs articulating to the centrum of the thoracic vertebrae
indicated by the costal facets is features differentiated during the early stage of adaptation to the aquatic life. In view of the fact that the costal facets of the thoracic
vertebrae of Delphinoidea and Physeteroidea locates on the posterior edge of thecentrum, the position of that of lniidae probably situates opposite that of all otherliving odontocetes. The synapomorphous characters of Lipotidae, Pontoporiidae
and Platanistidae are the development of the bony plate of pterygoid hamuli andthe reduction of the carpals. Through the development of the recurved thin platefrom the irregular small tubercles of pterygoid hamulus and the formation of theformer to the well-developed lateral plate of pterygoid, the evolutionary sequencesof this feature can be seen. The evolutionary trend of the carpals is the fusion of
the ulnare and pisiform to adjacent bones or missing of both bones.
The Lipotidae is the second branch descended from the primitive Platanistoids. A series of features of the Lipotidae such as the bony plate of pterygoid
hamuli, costal facet of thoracic vertebrae, carpals, zygomatic process of squamosaland crown of teeth is relatively more primitive than those of Pontoporiidae andPlatanistidae. The Lipotidae also differs from these two families by the apomorphous character in having the hepato-pancreatic duct opens into the duodenal
ampulla.
The Pontoporiidae and Platanistidae have diverged from the primitive Platanistoids slightly later than the Lipotidae. The maxillary crest, upper toothrows, carpals and apical bronchus of Platanistidae are characters more specialized
than those of the Pontoporiidae, whereas the caecum and the direct joining of the
tympano-periotic to the skull in the same family are primitive characters as compared with those of the Pontoporiidae. The mosaic distribution of the Plesiomorphous characters and apomorphous characters between Pontoporiidae and Platanistidae also has been formed.
The structure of the digestive tract of lniidae remains relatively close to theancestor. In Lipotidae and Pontoporiidae the fore-stomach is absent, the stomach of the former is most specialized in the losing of the fore-stomach and thedividing of the main stomach into three compartments. The digestive tract ofPlatanistidae is characterized by the differentiation of the main stomach and notable shortening of the intestine on the one hand, and is unique among odontocetes
in retaining the caecum on the other hand. The transverse crescent shaped blowhole of lniidae and Pontoporiidae is similar to that of most of the recent odontocetes,
whereas the shape of the blow hole ofLipotidae and Platanistidae has been changed
separately. The process of moving of the apical bronchus from the right bronchus
to trachea hasoccured in the course of phylogenetical development. The degreeof forward shift of the apical bronchus varies in different group of Platanistoids.
The reduction of the visual apparatus has taken place in relation to the increaseof the turbidity of the water and the weakening of the light. The eyes of theSci. Rep. Whales Res. Inst.,
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CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEA

105

Pontoporiidae are comparatively well-developed and those of the fresh water
Platanistoid species exhibit a marked regression. The eyes of the Platanistidae
are almost completely blind. The degree of reduction of the eyes is in order ofPontoporiidae, Lipotidae, Iniidae, then Platanistidae (Zhou, Pilleri and Li, 1980).
The orbit of Platanistoids is reduced in correspondence with the regression of theeye. In the case of Iniidae and Lipotidae, the zygomatic process of the squamosalis not lengthened in correspondence with the reduction of the orbit and is not in
contact with the supra orbital process of the frontal. The unique maxillary crest of
the Platanistidae functions together with the air sinuses in the reflection of the soundsignals. It was probably developed parallel with regression of the eye to compensate for the loss of vision (Pilleri, 1979). The development of the brain of the
Platanistoid families is comparatively low as compared with that of other living
odontocetes. In totality, the lniidae is the family keeping relatively more primitive characters and the Platanistidae is the most specialized one. The systematicsequence of the families in accordance with their evolution should be Iniidae,
Lipotidae, Pontoporiidae and Platanistidae.
According to the fossils of the primitive Platanistoids discovered, these four
·families probably diverged during the Miocene from the marine ancestor originated from the Squalodontoidea. Three of the recent families of Platanistoids
have entered fresh-water rivers secondarily, whereas Pontoporiidae has remainedin coastal waters. Although the structures of Platanistoids have been differentiatedduring the long period of evolution, their development level is still lower thanthat of other recent odontocetes. Further studies on fossil groups of the Platanistoids
are needed.
EVIDENCE OF THE MONOPHYLY OF THE CETACEA

Whether the cetacea originated from a common ancestor or two or several ancestors is a disputed problem. Slijper (1962) and some Soviet investigators havereached a conclusion in favour of polyphyly that the three suborders of the cetacea
probably originated separatly from respective terrestrial ancestors. On the contray, Gaskin (1976) and van Valen consider that the cetacea as a whole are probably monophyletic. The basis of their argument is a series of morphological characters which are common to mysticetes and odontocetes such as the loss of thepelage, the lacking of the true vocal cord, the similarity in the structure of the
tympanic bulla and the shape of the lung, the oblique position of the diaphragmand the moving of the blow hole to the dorsal aspect of the head, etc. The cytogenetic studies of Amason (1969, 1972, 1974) and Kulu (1972) are also quoted bythem to indicate the close agreement of mysticetes and odontocetes in the number
and shape of the chromosomes as well as the distribution of C-heterochromatin in
the chromosomes. No reasonable explanation would be obtained if the cetaceans
were not of common ancestry.
A character common to mysticetes and odontocetes, which has not beenbrought to great attention yet, has been noticed during the studies on the phylogeny
Sci. Rep. Whales Res. Inst.,
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106

ZHOU

of the Platanistoids. That is, the stomach of most cetaceans is composed of threecompartments, the fore-stomach, main stomach and pyloric stomach. The forestomach covered with esophageal epithelium is not subdivided further and theglandular stomach is divided into two or more compartments. The cetacean
stomach composed of single chambered fore-stomach and subdivided glandular
stomach is different from that of any other mammals. This fact makes me tocome over to the side of the monophyly.
The mysticetes are grazers and the odontocetes are predators. Since theydiffer not only in feeding methods but also in food, the similarity of the stomach
could not be explained by parallel adaptations. If the cetaceans were not ofcommon origin, it would be impossible for majority of them to possess the forestomach formed by the esophagus and the subdivided glandular stomach. It isreasonable to infer that such basic structure was developed before the differentiation of the mysticetes and odontocetes. It is the symplesiomorphous characterof the two groups and another evidence of the monophyly of the cetacea.
Certain characters which were thought to be found in mysticetes only still
remain in some Platanistoids, such as the hair on the snout of Iniidae and the caecum of Platanistidae. These features add evidence to support the monophyly of
the cetacea. It should be stated that the unique musculus palpebrales of the cetaceans (Hosokawa, 1951) is also a proof which has not been cited by previous authors.
ACKNOWLEDGEMENTS

The present paper was completed after I had examined the Platanistoid skeletons deposited in the Ocean Research Institute of the University of Tokyo. I am greatly
indebted to Dr. M. Nishiwaki, Professor Emeritus of the University of Ryukyus,
for his kind invitation made my studies in Japan possible, and to Dr. T. Kasuyaof the Ocean Research Institute of the University of Tokyo who provided theabove mentioned specimens in his collection. Thanks are also due to Xu Xinrong
for drawing.
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ARNASON, U., 1969. The karyotypes of the fin whale. Hereditas, 62: 273-84.
ARNASON, U., 1972. The role of chromosomal rearrangement in mammalian speciation with special reference to Cetacea and Pinnipedia. Hereditas, 70: 113-18.
ARNASON, U., 1974. Comparative chromosome studies in Cetacea. Hereditas, 77: 1-36.
ARVY, L. and G. PILLER!, 1970. The tongue of Platanista gangetica and remarks on the cetacean tongue.
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BREE, P.J.H. van and P. E. PuRVEs, 1975. On the dimensions of three skulls of the species of dolphin
Lipotes vexillifer Miller, 1918 (Cetacea, Platanistoidea, Iniidae). Beaufortia, 24(308): 1-5.
BROWNELL, R. L. Jr. and E. S. HERALD, 1972. Lipotes vexillifer. Mamm. sp., 10: 1-4.
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CLASSIFICATION AND PHYLOGENY OF PLATANISTOIDEA

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