IO96 CLASS REPTILIA. 



vertebras would also seem to be a generalised character ; and, from the 

 evidence afforded by the above-mentioned Pleurostemu?n and Archceo- 

 chelys, the presence of an intergular shield should probably be likewise 

 regarded in the same light. The anchylosis of the pelvis to the plastron, 

 as pointed out by M. Dollo, would, however, seem to prevent us from 

 regarding the Pleurodira as the ancestors of the Cryptodira ; and it is 

 more probable that both sections should be regarded as divergent 

 branches of a common stock, probably represented by Amphichelydia of 

 earlier age than those yet known. 



It would seem probable that the palate of the earlier Mesozoic Pleuro- 

 dirans approximated to the Cryptodiran type, and that the peculiar man- 

 dibular articulation and closed tympanic ring of the existing forms are 

 acquired features. The skull of the Mesozoic types appears generally to 

 have been roofed over after the fashion of the modern Turtles ; this struc- 

 ture having apparently been common to many of the earlier forms of both 

 this and the following sections of the suborder. 



Family Proganochelyid^e. — This name is proposed by Dr 

 Baur for the reception of the genus Proganochelys, from the Keuper 

 of Wiirtemberg, which he regards as a Pleurodiran, although differ- 

 ing from all other members of the section in that the plastron unites 

 only with the edges of the marginals, without giving off axillary and 

 inguinal buttresses. It is suggested that mesoplastral bones were 

 present, while the carapace is compared to that of Platychelys. 

 The latter resemblance suggests that this form might belong to the 

 Amphichelydia, but a study of the figure of the type specimen given 

 by Professor Quenstedt, under the synonym of Psammochelys, shows 

 that its extremely imperfect nature scarcely permits any decisive 

 opinion to be formed as to its true affinities. Whether the imper- 

 fectly known and apparently aberrant genus Chelytherium of the 

 Keuper is an allied form cannot yet be determined. 



Family Plesiochelyid^e. — This Mesozoic family is characterised 

 by the total absence of the mesoplastral element in the plastron, and 

 also by the circumstance that the pubis alone is united with the 

 xiphiplastral ; while there may or may not be a complete series of 

 neural bones articulating posteriorly with the suprapygal. The plas- 

 tral shields are separated from the marginals by the intervention of 

 a series of inframarginals ; and the plastron, as in the succeeding 

 families, is connected by long buttresses with the carapace. The 

 humerus is of the type found in existing Pleurodira ; and the second 

 digit of the manus, when known, has but two phalangeals. The 

 skull is known only in a few cases with certainty. 



The typical genus Plesiochelys and the closely allied Crasftedochelys 

 were originally described from the Lower Kimeridgian Lithographic 

 limestones of Bavaria and France. These genera have a thick shell, 

 with the full complement of neural bones, which posteriorly join the 

 first suprapygal, and with the vertebral shields of the carapace of 

 moderate width. The entoplastral bone is relatively wide and 



