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Show 172 DR. A. S. WOODWARD ON EXTINCT [Mar. 5, of the orbit, which is a little longer than deep and of about the same size as the narial opening. The tympanic opening (au.) is almost quadrilateral and equilateral, with rounded angles. It is scarcely more than half as large as the orbit, and is specially remarkable as being bounded behind by a gently convex, thick plate of bone, which is as broad as deep. This bone is only preserved on the right side of the fossil, but even here is too imperfect for the determination of its constitution. It probably represents the hinder part of the quadrate and squamosal fused with a dermal bone. Viewed from below (Plate XVI.), the cranium proper is seen to be a little broader than long, with the snout rounded, and a slight constriction of the sides below the orbits. The palatal expansion of the maxillae and premaxilke bears a large inner rounded ridge (r.) concentric with the comparatively acute margin of the jaw, thus indicating the herbivorous nature of the animal. The roof of the mouth between this tritural border is raised into a dome with the concavity downwards. The posterior nares (p.na.) thus face backwards rather than downwards, and are separated by a broad flattened bar, which seems to be formed by the premaxilla? in front and by the vomer behind. Immediately behind the posterior nares, the antero-posteriorly arched area, which appears to be an unusually large vomer, bears a sharp median longitudinal keel. The limits between all the elements are uncertain, but the palato-pterygoid region (pt.) of the palate is relatively very short and broad, while the lateral margin of the pterygoid, though incomplete on both sides, exhibits no trace of the curious rolled-up lateral process so characteristic of existing Pleurodira. The pterygoids clearly meet in the middle line, and there is a small, transversely extended interpterygoid vacuity (i.p>t.). The relatively small basisphenoid (b.s.) is not quite in the same plane as the basioccipital, but inclines a little upwards in front. A long narrow buttress extends outwards from it on either side to the portion of the quadrate bone which would bear the articulation for the mandible ; but this region is too imperfect for precise interpretation. The buttress is fused postero-superiorly with the otic bones (ot.), which form a great mass at the sides of the occiput in which no sutures are discernible. The basioccipital is concave on its lower face ; and the occipital condyle, of uncertain constitution, is about twice as broad as deep. The foramen magnum is slightly deeper than broad, but comparatively small. The median crest of the supraoccipital (s.occ.) is a slender lamina, somewhat expanded above where it fuses with the roof of the temporal fossae. Neither this nor any of the otic bones extend backwards beyond the plane of the occipital crest. The inner part of the cranium is well divested of matrix, and two other interesting features are thus exposed. The hinder part of the pterygoid is connected with the parietal region by a small vertical lamina of bone on either side. The nasal cavity is completely separated from the orbit on each side by a thin bony |
