44 FOSSIL TURTLES OF NORTH AMI- RICA. 



Skull essentially cryptodiran in structure, but with various primitive elements. Neck short, 

 the vertebrae little differentiated. Limbs, so far as known, fitted lor walking. 



This group was establisht by Mr. Richard Lydekker, to include, as he states, 

 a number of generalized late Mesozoic forms which may be regarded as allied to the 

 earlier (and then unknown) progenitors of the Pleurodira and Cryptodira. He 

 characterized them as having a shell constructed on the plan of" that of the Crypto- 

 dira and Pleurodira, in which mesoplastral bones and an intergular shield are 

 developt. The pubis may articulate, without sutural union, with the xiphiplastron. 

 At the time he wrote the skull and neck were not known. The coracoid and the 

 humerus were regarded as being of the pleurodiran type. The genera included 

 were Pleurosternon, Ba'ena, and Platychelys. (Lydekker, Cat. Loss. Rept. Brit. 

 Mus., pt. in, 1889, p. 204.) 



In 1890 (Amer. Naturalist, xxiv, p. 530) and 1891 (Proc. Acad. Nat. Sci. Phila., 

 p. 411) Dr. George Baur, working on materials at Yale University, added greatly 

 to the knowledge of the group, regarded by him as a suborder. He likewise furnisht 

 a definition of the group. The present writer (Bull. Amer. Mus. Nat. Hist., xxi, 

 1905, p. 137). having studied well-preserved remains of Ba'ena from the Bridger 

 beds, was enabled to make further additions to our knowledge of the Amphichelydia 

 and to correct some of" Dr. Baur's statements and generalizations. 



In this superfamily the plastron appears to be in all cases more or less closely 

 joined to the carapace by sutural union with the peripherals, and in the Baenidae 

 the union is strengthened by powerful axillary and inguinal buttresses. In all 

 forms known to belong to the group there are well-developt mesoplastra; but since 

 these bones have been abohsht from the plastron of all the Crvptodira and from 

 that of most of the Pleurodira, there seems to be no good reason to suppose that they 

 might not have been supprest in the case of some of the Amphichelydia. Accord- 

 ingly the writer has ventured (Bull. Amer. Mus. Nat. Hist., xxi, pp. 144, 167) to 

 include provisionally in the superfamilv the Plesiochelvida?. Nothing is known 

 certainly of the members of the family except the shells. 



On the carapace of the Amphichelydia, besides the scutes found in such turtles 

 as the hmydidae, there may occur supernumerary vertebrals, costals, and supra- 

 marginals. On the plastron there are, in all known cases, intergulars and full series 

 of inframarginals. The boundaries between the scutes are liable to vary in their 

 positions. The sulcus between the paired scutes of the plastron especially is often 

 found wandering far from the midline. It is as if the limits of the scutes had not 

 in these early forms yet become firmly establisht. In the skull there are such primi- 

 tive elements as distinct nasals and lacrimals. The temporal region is always 

 covered over and the quadrates are notcht for the passage of the stapedial rod. 

 The pterygoids never have the wing-like expansions that characterize the Pleuro- 

 dira; and, unlike the latter, the pterygoids push themselves backward between the 

 quadrates and the basioccipital and the basisphenoid. 



The cervical vertebrae may, as in Glxptops, be all biccelous, or, as in the Baenidae, 

 one end of most of them may be convex. The neck was short and the differentiation 

 of the cervicals had progrest so little that probably these turtles could protect their 

 heads neither by withdrawing them within the shells as do the Emydida;, nor by 

 laying them on one side beneath the front border of the carapace after the manner 

 of the Pleurodira. 



In the Pleurosternidae and the Baenidae, whose limbs are now known, these were 

 fitted for progression on land. Doubtless they were inhabitants of the waters of 

 lakes and rivers and were good swimmers, but there was no special modification of 

 the limbs tor swimming. 



