44 FOSSIL TURTLES OF NORTH AMERICA. 
Skull essentially cryptodiran in structure, but with various primitive elements. Neck short, 
the vertebra little differentiated. Limbs, so far as known, fitted for w alking. 
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 een unknown) progenitors of the Pleurodira anu 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, Baéna, and ”Platyc helys. (Lydekker, Cat. Foss. Rept. Brit. 
Mus., pt. 111, 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 asa suborder. He likewise furnisht 
a dehnition of the group. Lhe present writer (Bull. Amer. Mus. Nat. Hist., xx1, 
1905, p- 137), having studied well-preserved remains of Baéna trom 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 en in all cases more or less closely 
joined to the carapace by sutural union with the peripherals, and in the Baénidz 
the union 1s strengthened by powerful axillary and inguinal buttresses. In all 
forms known to belong to the group there are w ell-dev elopt mesoplastra; but since 
these bones have been abolisht from the plastron of all the Cryptodira 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., xx1, pp. 144, 167) to 
include provisionally in the superfamily the Plesiochelyida. Nothing 1s known 
certainly of the members of the family except the shells. 
On the c carapace of the Amphichely dia, besides the scutes found in such turtles 
as the Emydide, 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 jiable 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 1s 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 fhe basisphenoid. 
The cervical vertebra may, as in Glyptops, be all biccelous, or, as in the Baénidee, 
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 Emydidz, nor by 
laying them on one side beneach the front border of the carapace after the manner 
of fhe Pleurodira. 
In the Pleurosternidze and the Baénidz, 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 for swimming. 
