OCR Text |
Show 1895.] SKULL OF OSTEOGLOSSUM FOBMOSUM. 303 alisphenoid (as.) bones at the point where the three are in sutural connection with one another near the antero-superior angle of the auditory capsule (fig. 1). This process is, undoubtedly, the equivalent of the conspicuous ascending or lateral process of the parasphenoid in certain Ganoids, which it also closely resembles in its extensive relations to the anterior wall of the auditory capsule, and, more particularly, in its extension so far dorsally as to overlap the sphenotic region-an extension which, so far as I a m aware, has no parallel in any other Teleost, although characteristic of such Ganoids as Acipenser, Polypterus, and Amia. The second of the two processes (figs. 1 and 2, ps.a.) is about 13 m m . in length and grows out on either side from the lateral margin of the parasphenoid, immediately ventrad to the root of the ascending process, and tapers to a free distal extremity. The process is horizontally disposed, at right angles to the long axis of the parasphenoid, or, at any rate, has but a very slight upward inclination from its root outwards, and is furnished with smooth rounded surfaces. This process may be termed the articular process of the parasphenoid. Midway between the roots of the two articular processes, and projecting downwards from the ventral surface of the parasphenoid, there is a mesially situated tubercle of bone (fig. 1, ps.t.) which supports a small cluster of teeth. These are small but variable in size, conical in shape, with pointed crowns, and so arranged that the two largest teeth occupy the hinder margin of the cluster, while about six smaller teeth are closely grouped together immediately in front of them. The formation of special articular processes in connection with the parasphenoid is correlated with certain modifications in the mandibular arch, whereby the latter acquires a special articular connection with the former. The metapterygoid (figs. 3 and 4, mt.pg.) occupies its normal position as the proximal element of the arch. Posteriorly, the bone overlaps the external surface of the inferior half of the hyomandibular (hym.) and the contiguous outer surface of the symplectic (sym.); anteriorly, the metapterygoid is, in turn, overlapped on its inner surface by the mesopterygoid (m.pg.), while inferiorly it is firmly articulated to the superior margin of the quadrate (q.). The mesopterygoid (fig. 3, m.pg.) is the largest of the palato-pterygoid series, of which it forms the upper two-thirds. It is somewhat triangular in shape, the apex being directed anteriorly towards the palatine region, while the wider posterior portion overlaps the inner surfaces of the metapterygoid and quadrate bones. Inferiorly, the_ bone is firmly united to the pterygoid (pg.) throughout its entire length. The pterygoid (pg.) forms the inferior third of the palato-pterygoid series of bones, overlapping the inner surface of the quadrate behind, while externally, and for at least the anterior half of its length, it is rigidly attached to the inner surface of the dentigerous maxilla. A distinct palatine element is either non-existent or entirely cartilaginous, at any rate no trace of a palatine bone could be detected in the specimen examined. |