The Homalopterid Skull
Authors: Ramaswami L. S. 
Published: August 2010

Summary.

The skull of the homalopterid fishes is generally flattened and is compactly built.
The upper lip is supported by a set of two pairs of bones in Bhavania and Balitora (laerimojugal and rostral) and a single large one in Gastromyzon (lacrimojugal‐rostral).
The median limb of the premaxilla in Bhavania and Balitora sits on the median rostral, which is an ossification in the fused anterior upper labials; in Gastromyzon the median limbs are anterior to the cruciform median rostral.
From the ethmoid, with which probably the supraethmoid is also fused, processes are given off anterolaterally for articulation with the preethmoid bones, long and cylindrical in Bhavania and Balitora. Sitting upon these pre‐ethmoids, is the lateral rostral in Bhavania and Balitora; in Gastromyzon the lateral rostral articulates anteriorly to the pre‐ethmoid.
The lateral ethmoids are large bones supporting lacrimojugals or lacrimojugal‐rostrals.
There is a supraorbital sensory canal passing in the frontal and nasal; only in Gastromyzon is there an independent supraorbital sensory canal bone near the sphenotic. The suborbital sensory canal passing through a series of tubular sensory bones ends in the rostral (Bhavania and Balitora) or beside the lacrimojugal‐rostral as in Gastromyzon. In the latter genus there is also a set of four mandibular canal bones starting from the postorbital. In Bhavania and Balitora the sensory canal runs partly in the pterotic bone; by the side of the sphenotic there are two sensory canal ossicles in Bhavania and one in Balitora, while in Gastromyzon there are four ossicles by the side of the sphenotic and pterotic, and no part of the sensory canal passes through the latter bone.
Behind the pterotic, both in Bhavania and Balitora, there are three extra‐scapulars, two of which are canaliculated; in Gastromyzon there is only one extrascapular corresponding to the lateral extrascapular of the other forms leading the lateral line sensory canal.
Of the otic bones, the prootic, pterotic, sphenotic, and epiotic are separate ossifications; the opisthotic seems to have merged with the exoccipital. It is not known if the pterotic represents a compound bone with the sensory canal bone merged with it in Bhavania and Balitora.
The broad post‐temporal comes in contact with the posterior face of the pterotic and with the upper surface of the cleithrum.
The cleithrum comes in intimate contact with the enlarged first vertebra and the latter is also firmly articulated with the occipital condyles.
A preopercular is absent in Bhavania and Gastromyzon; in Balitoraa preopercular and a tubular sensory canal bone in front of it are present, but a symplectic is absent. A hitherto unnoticed bone in contact with the posterior limb of the quadrate in the upper jaw of modern fishes, and called by me the “posterior ectopterygoid” and generally articulating with the metaptery‐goid is recorded in all the three genera of Homalopteridae studied.
While a subtemporal fossa is prominently noticed in Bhavania and Balitora, it is absent in Gastromyzon.
A retroarticular is prominently noticed in the lower jaw of Bhavania and Balitora; in Gastromyzon a retroarticular is absent and a sesamoid angular (sesamoid articular) is well developed. In front of the sesamoid angular in Gastromyzon there is a small scale‐like sesamoid bone, which is, however, absent in Bhavania and Balitora.