BAENIDZ. 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 Emydida, 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 trom 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 1s ee area occupied in the Emy didzx by the first renenral: 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 Baéna (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 
Baéna. 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 Chisternona 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; aio, 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 hay e been supprest. In H. von Meyer’s representation of Acichelys 
crassipes (his Paleomedusa testa, Lithogr. Schiefer, pl. xx, fig. 1) 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 Baénide 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 
