TRioNYcnoinEA. 239 



anterior vcrtcbroo by a small tubercle below the proczyj^-apopliyses. 

 The ncuro-central suture is persistent. All tlie vertebnu except the 

 first and eighth arc opisthoccolous, and the articulation is ging-lymoid 

 between the seventh and eighth. 



Sacral and Caudal Verxebk.e. — The sacral vertebra) arc two in 

 number, with very strong ribs suturally united together distally. 

 As a rule, the sacral ribs articulate with the neural arch onlj^ ; 

 Pelocliehjs forms, however, an exception, the ribs articulating with 

 both arch and centrum, as in other Chclonians. The caudal ver- 

 tebrae are prococlous, with strong transverse processes which, in 

 the anterior vertebra), are entirely upon the neural arch. The ncuro- 

 central suture persists on the anterior vertebra). They range iu 

 number from 14 to 19. Chevron bones are absent. 



Skull. — The skull, which is much depressed, is remarkable for the 

 great development of three crest-like posterior processes, dii'ccted 

 backwards and upwards, viz. the supraoccipital and, on each side, the 

 squamosal. The orbits are moderate or small, turned obliquely 

 upwards, and in some forms (especially Chitra) close to the anterior 

 extremity of the skull ; they are bordered by the maxillary, the pra)- 

 fi'ontal, the frontal, the postfrontal, and the jugal. The praemaxillary 

 is extremely small, single, not entering the nasal opening, and widely 

 separated from the vomer, the maxillaries uniting in a mediaii 

 suture in front of the choana). The pra)frontals are large and in 

 contact throughout, and constantly separated from the postfrontals 

 by the frontals ; the postfrontals are comparatively small, and the 

 parietals join the jugals, the two latter bones forming a suture on 

 the npper surface of the skuU in such forms as have a broad post- 

 orbital arch. The parietals, which have large descending palatal 

 processes, never expand into a supratemporal roof. A zygomatic or 

 temporal arch is present, but constantly rather slender, and formed 

 by the jugal and the quadratojugal ; the latter bone is small, and 

 in Chitra and Pelocliehjs separates the jugal from the squamosal. 

 There is no parieto-squamosal arch. The quadrate surrounds the 

 outer ear-chamber, which is completely closed, with the exception 

 of a small foramen for the passage of the stapes; the tympanic 

 border is formed by the squamosal above, and a rather shallow notch 

 is present in the quadrate posteriorly. Two very distinct articular 

 facets are present on the quadrate, corresponding with tlie con- 

 cavities of the mandible. 



The structure of the palate is very characteristic. The vomer is 

 small, separates the choanse, and is usually connected by ascending 

 processes with the pra)frontals. The palatines are large and form a 

 median suture ; they are posteriorly in extensive contact with the 

 basisphenoid, which is more developed than in any other Thecojjhore, 

 and widely separates the pterygoids. The latter bones are broad, 

 with straight or convex outer borders forming more or less distinct 

 wings ; they are devoid of lateral processes, and extend posteriorly 

 beyond the quadrate, which they sej)arate from the basioccipital. 



In Trionyx and Emyda the opisthotic is produced posteriorly into 

 a crest applied against the squamosal process. 



