ORDER CHELONIA. 



IO99 



V-fr- 



Fig. 1014 bis. — The cranium of 

 Rhinockelys cautabrigicnsis, imper- 

 fect posteriorly ; from the Cambridge 

 Greensand. /«, Parietal ; pt.fr, 



chelys. It has been suggested that this genus is Pleurodiran ; and 

 if this prove to be the case it will be interesting as showing an ap- 

 proximation in several cranial features to the Cryptodira. In its 

 complete roof the skull of this genus approximates to the Chelonida. 

 Remains of Rhinochelys are very abundant 

 in the Cambridge Greensand, and they 

 are also met with in the Gault and the 

 Lower Chalk. Fragmentary Chelonian 

 shells from the Cambridge Greensand, 

 to which the name Trachydermochelys 

 has been applied, are not improbably 

 referable to this genus. They are char- 

 acterised by their pustulate external sur- 

 face ; the pustules being much larger 

 than in the Amphichelydian genus Helo- 

 chelys of the Lower Greensand. A 

 somewhat similar, although less marked, 

 pustulation occurs in the existing Chelo- 

 dina. 



Family Pelomedusid.e. — Reverting 

 to the consideration of undoubted Pleu- l^^'^^ifHiS] 

 rodirans the present existing family agrees P mx ^ Premaxiiia. 

 with the Amphichelydian Pleiirosternida 



in the presence of mesoplastral bones, but is distinguished in 

 that both the pubis and ischium have a sutural union with the 

 xiphiplastral. The shields of the plastron, as in the next family, 

 are in contact with the marginals, owing to the absence of infra- 

 marginals. The skulls of existing types have an infratemporal 

 arcade, and in Podocne?nis, alone among existing representatives 

 of the section, the temporal fossae are roofed over ; moreover, the 

 prefrontals are in contact in the middle line and are fused with 

 the nasals ; the palatines, owing to the absence or abortion of 

 the vomer, meet ; and the suture between the dentary bones of 

 the mandible is obliterated. The second cervical vertebra is 

 amphiccelous ; and the neck is completely retractile within the 

 shell. Further, the series of neural bones, as in the next family, is 

 incomplete and is not connected with the suprapygals. Finally, in 

 both this and the next family the characters of the tympanic and 

 palatal regions of the skull are those mentioned at the commence- 

 ment of the description of the Pleurodira. The earliest known 

 representatives of this family occur in the Upper Cretaceous of the 

 United States, where we meet with forms apparently closely allied 

 to the existing Podocnemis. The genus Bothremys has been described 

 upon the evidence of a skull from those deposits ; while the name 

 Taphrosphys has been applied to portions of the carapace and 



