FOSSIL AND RECENT 17 



myodome. The parasphenoid is produced dorsally as a shallow ascending wing 

 which partially overlaps the prootic. A foramen for the internal carotid artery 

 occurs within the parasphenoid at the base of the ascending wing. Posteriorly 

 the parasphenoid ends in a notch forming the lower edge of a foramen leading to the 

 myodome, the dorsal edge of this foramen being formed by the basioccipital. Beneath 

 the orbit the parasphenoid bears teeth. In young individuals the teeth are very 

 small and clustered along the margins of the bone but in adults the pattern changes 

 to a dense and complete covering of small villiform teeth. The parasphenoid is 

 usually pierced in the mid-line beneath the basisphenoid by a foramen for the bucco- 

 hypophysial canal but in the specimen figured here (Text-fig. 5) it is closed ventrally. 



The vomer consists of a broad head and a spear-shaped shaft which extends 

 posteriorly beneath the lateral ethmoid. The paired vomerine tooth plates are 

 raised upon short pedicels. In small individuals each toothed area is separated 

 from its partner and bears about 20 small recurved teeth. During growth the 

 number of teeth increases to about 60 and the toothed areas of either side approxi- 

 mate to one another in the mid-line. Even in the largest specimens examined the 

 toothed area retains its identity as a paired element. 



Much of the snout region is formed by cartilage. Between the dermethmoid 

 above and the vomer below the ethmoid cartilage separates the nasal capsules 

 of either side. The posterior, ventral and anterior walls of the nasal capsule are 

 composed of ethmoid cartilage. Beneath the anterior region of the solum nasi the 

 cartilage is somewhat denser than elsewhere and forms an oval facet for the articula- 

 tion of the palatine head. Two facets for jaw articulation occur upon the lateral 

 face of the cartilage above the vomer. The posterior of these receives the head of 

 the maxilla while the anterior receives the premaxilla. In the largest individuals 

 the extreme anterior tip of the ethmoid cartilage may show a limited amount of 

 endochondral ossification. Posteriorly the lateral edge of the planum antorbitale 

 ossifies perichondrally to form the lateral ethmoid. The olfactory tract and the 

 nasal branch of the orbitonasal artery pierce the planum antorbitale through a 

 large foramen olfactorium advehens. There is no anterior myodome. 



Hyopalatine bones. The hyomandibular is a stout element with a single broad 

 articulatory head (a distinction from the type-species which shows a weakly divided 

 head, Ridewood 1904) which is inclined antero-ventrally. The hyomandibular is 

 produced postero- ventrally as a narrow shaft. Dorsally the posterior margin of 

 the hyomandibular projects as a prominent opercular process which articulates with 

 a cup-shaped depression on the operculum. The hyomandibular ramus of VII and 

 the efferent hyoidean artery pierce the hyomandibular immediately beneath the 

 anterior limit of the head and run obliquely through the bone to emerge beneath 

 the opercular process before running down in a groove on the posterior edge of the 

 shaft. Anteriorly the head of the hyomandibular is produced as a thin wing of 

 bone which lies medial to the dorsal part of the metapterygoid. The cup-shaped 

 space left between the hyomandibular and metapterygoid, the hyomandibular- 

 metapterygoid cup, receives deeper fibres of the levator arcus palatini. 



The metapterygoid is irregular in shape. Its dorsal margin is turned horizontally, 

 medial to that part referred to above which overlaps the hyomandibular. The 



