Halystina conoidea
Creators: name: Helwerda, Renate Ariane
name: Wesselingh, Frank Pieter
name: Williams, Suzanne T.
Publisher: name: Zenodo
Year: 2014
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Gastropoda
Seguenziida
Seguenziidae
Halystina
Halystina conoidea

description: Halystina conoidea nov. spec.
(Figures 13–15)
Type material. Holotype RGM 608.229. Paratype RGM 608.230 (Anda 6); 7 paratypes RGM 608.235 (AndaDeVos).
Type locality. AndaDeVos, Bolinao, Philippines, Santa Cruz Formation, Pliocene or Early Pleistocene.
Derivatio nominis. Named after the conical outline of this species. Adjective.
Other material. Anda 5 (1); Anda 6 (2); AndaDeVos (7); AndaClif3 (4); Tiep 1 (1).
Diagnosis. Shell conoid, H 2.1 mm, W 1.7 mm, DN 0.11–0.13 mm, low spiral ribs, thin axial lirae; axial lirae gradually disappearing; two slightly thicker but low spiral ribs at middle of whorl, just above suture; after second teleoconch whorl, third spiral rib appears just below suture; body whorl with additional rib on periphery; 11 spiral ribs on the convex base; base of aperture with broad short siphonal canal.
Description. The protoconch is paucispiral and has rounded whorls and a relatively dull white surface. The protoconch surface in the studied specimens is corroded, but two spiral ribs appear to be present. The nucleus is slightly inclined. The P/T boundary is located at about 1.1 whorls. The teleoconch ornamentation starts with thin axial riblets that are low and narrow but well delimited. They have a reverse sigmoid shape and are most pronounced at the upper quarter of the whorl. Within 0.2 whorls from the P/T boundary, a very low and rounded spiral rib develops at half the height of the whorl that subsequently moves to two-thirds of the whorl height within half a whorl. This poorly defined rib delimits a low shoulder on the first and second teleoconch whorl. Already on the first teleoconch whorl, a second spiral develops at approximately one quarter of the whorl height. On the second teleoconch whorl, a narrow subsutural ridge develops that forces the suture into a narrow and shallow depression. The whorl profile becomes gradually flatter with the initial spiral rib becoming even lower and located at 0.6 of the whorl height and the basal rib at 0.2. In between the spiral ribs, the profile is straight or slightly concave. The axials (approximately 55–60 on the second teleoconch whorl) gradually disappear over most parts of the whorls, but can remain visible at the top of the whorls. The suprasutural spiral forms a low and broad median keel on the body whorl, together with a spiral rib on the periphery. At the slightly convex base of the body whorl, 11 more or less regularly spaced low spiral ribs occur, whose interspaces are about twice as wide as the ribs. The aperture is damaged and/or partially covered in all specimens, but is subtrapezoidal in juveniles and subquadrate in adults, where the outer lip is expanded basally. The outer lip also has a retraction in the upper part, between the suture and the periphery. The columellar lip is barely thickened, appears slightly twisted and grades into a broad and shallow basal sinus. A single specimen contains a very subtle columellar fold on the base of the columella. The umbilicus is rimate.
Differentiation. Halystina conoidea differs from H. vaubani Marshall, 1991 and H. caledonica Marshall, 1991, both Recent species from New Caledonia, in having axial ribs that become obsolete instead of being well developed throughout the teleoconch. It differs from H. carinata Marshall, 1991, also from New Caledonia, and H. simplex (Barnard, 1963) from South Africa by a more subtle shoulder. It resembles most closely H. globulus Poppe, Tagaro & Dekker, 2006, from the Philippines, but differs in the almost entirely closed umbilicus (instead of open). Halystina conoidea differs from H. edax Bertolaso & Palazzi, 1999, from the Pliocene of Italy, and H. siberutensis (Thiele, 1925), a Recent species from Indonesia, in having an angular periphery.
description: Published as part of Helwerda, Renate Ariane, Wesselingh, Frank Pieter & Williams, Suzanne T., 2014, On some Vetigastropoda (Mollusca, Gastropoda) from the Plio-Pleistocene of the Philippines with descriptions of three new species, pp. 101-135 in Zootaxa 3755 (2) on pages 110-111, DOI: 10.11646/zootaxa.3755.2.1, http://zenodo.org/record/250959