BAENID-ffi. 57 



a nuchal, 12 pairs of peripherals, and what must be regarded as a suprapygal, but no true 

 pygal. The plastron has, besides the bones found in the Emydidae, a pair of mesoplastrals, 

 which usually reacht the midline and expanded greatly outward to join the fifth and sixth 

 peripherals. The axillary and inguinal buttresses are high and wide. Between the axillary and 

 the inguinal of each side there is a large sternal chamber. The axillary buttress ascended to 

 a point a little above the lower border of the first costal, meeting there the comprest rib of 

 the first dorsal vertebra. The inguinal buttress rose above the lower border of the fifth and 

 sixth costals and was articulated in a ridge rising from the adjacent borders of these costals. 



The scutes of certain regions of the shell are extremely variable; in other regions they vary 

 little. On the carapace it is the area occupied in the Emydidae by the first vertebral, the nuchal, 

 and the first pair of costals that is subject to variation in its scutes. There may be on each side 

 a supernumerary first costal scute, or it may occur on only one side, or it may be wholly absent. 

 In some specimens is a second pair of supernumerary costals, lying just behind the nuchals. 

 There may even be a supernumerary vertebral cut off" from the front of the normal first 

 vertebral. 



On the plastron the gulars and intergulars are variable in form and size. The infra- 

 marginals vary in number and size and form. There are usually 4, but often the second from 

 the axillary notch is missing on one or both sides. In this case the pectoral scute may or may 

 not reach the marginals. 



In Chisternon there intervenes between the nuchal bone and the first neural a large bone 

 that is not present in Baena (fig. 76). This the writer calls the preneural. In this genus too the 

 occasional variations in the scutes of the front of the carapace appear to be quite the rule. The 

 normal first vertebral scute is transversely divided, so that there are 6 vertebrals. There is 

 usually on each side a supernumerary costal and sometimes two of them, making 6 pairs of 

 these. The same variations occur in the scutes of the plastron of this genus that we find in 

 Baena. In one case there are 5 inframarginals. 



It would be worth much to know the meaning of these variations in the bones and scutes. 

 Is the presence of the preneural in Chisternon a. primitive or a secondary condition ? The writer 

 does not regard it as probable that a bone like the nuchal has become secondarily divided, or 

 that a new bone has become developt in that region. It appears more probable that both it and 

 the nuchal are continuations forward of the row of neural bones, and that in the most 

 advanct turtles the preneural has been crowded out of existence by the growth of the nuchal. 

 It is evidence in favor of this view that in Boremys Lambe there is present a small preneural; 

 also that it is present in the trionychoid genera Aspideretes and Plastomenus. These genera 

 have probably inherited this bone from their Amphichelydian ancestors. 



As regards the supernumerary scutes, we have the same questions to answer as in the case 

 of the prenuchal. Do we have here a breaking up of the normal scutes into smaller areas, as has 

 been observed in some modern genera by H. Gadow, W. P. Hay,and R.E. Coker; ordo we have 

 the normal number of scutes that were present in the earlier turtles, together with a tendency 

 to a suppression of some of them ? We must recognize the fact that on most parts of the shell 

 the scutes are as stable as in ordinary turtles; altho, especially on the plastron, there is a ten- 

 dency for the sulci to wander somewhat wildly. It is to be noted that the variations occur in 

 those regions where, in modern turtles, certain scutes have been supprest. There can be no 

 doubt that intergulars were primitively a possession of all turtles, but in most Cryptodira they 

 have been supprest. The primitive turtles likewise possest on each bridge a complete row of 

 inframarginals; but in most living genera these have been eliminated, with usually the excep- 

 tion of an axillary and an inguinal scute. There is probably not so much evidence that one or 

 more anterior costal scutes have been supprest. In H. von Meyer's representation otAcichelys 

 crassifes (his Palaomedusa testa, Lithogr. Schiefer, pi. xx, fig. i) there is shown a pair of super- 

 numerary costal scutes. It is also interesting to observe in that specimen 2 neurals between the 

 first pair of costal bones. The anterior of these is probably the preneural. In the modern 

 genus Caretta there are 5 pairs of costal scutes. It appears probable that the preneural and 

 the supernumerary anterior vertebral scute tended to disappear together. 



The skull of the Ba'enidce presents various primitive features. The presence of distinct 

 nasals, of lacrimals, and a wide temporal roof is to be cited. The writer has not found 

 epipterygoids. The short supraoccipital is likewise primitive. When we leave aside these 



