350 
also in Staurotypus, while L. Agassiz (Contributions to the Nat. Hist. of 
the U. S. A., vol. I, p. 267) states that it disappears in old specimens of 
other Cinosternidze. With these exceptions the entoplastron occurs as a 
single median bone in all known species of turtles and tortoises both liv- 
ing and fossil, save where in some of the latter the fragments are too 
meagre to permit its presence or absence being positively determined. It 
is therefore phylogenetically a very old element in the testudinate skeleton, 
and was probably, in some form or other, a direct inheritance from the 
more generalized reptilian stock from which this order arose. 
It follows, therefore, that we have in the paired entoplastron of the 
embryo Trionyx, a very primitive character, so primitive, indeed, that it 
occurs nowhere in the adult of any known species of Testudinata either 
living or fossil. It is therefore an indication that T'rionyr is to be re- 
garded as more primitive than any other known genus of the order. Were 
this the only evidence of primitiveness known to occur in Trionyr, one 
would not, perhaps, be justified in making so broad an assertion. But a 
considerable amount of Gorroborative evidence is also at hand. Thus in 
Trionys, the atlas is temnospondylous, i. e., its three constituent parts, 
the neural arch, the centrum, and the intercentrum, are not ankylosed but 
remain loosely connected, there is no odontoid process on the second ver- 
tebra, the first centrum being freely movable on the second; the pubic 
and ischiadic symphyses are broad and are connected with each other by 
a longitudinal cartilaginous band, which is replaced in other testudinates, 
except Chelone, by a broad completely ossified plate (Gadow). In the 
young of all tortoises, but in the adult only in the Chelonidze and Tri- 
onychidie, the plastral plates are separated by large fontanelles (Fig. 1, f). 
And finally, as reported by Wiedersheim (Vergl. Anat. der Wirbelthiere, 
5. Auflage, 1902) teeth rudiments also occur in the embryo of T'rionyr and 
nowhere else among the Testudinata. I have not been able so far to ¢or- 
roborate this observation, but it is certainly, if correct, a most important 
argument in favor of the view herein set forth. ; 
This conclusion regarding the Trionychia is not invalidated by cox tian 
secondary specializations, such as the flatness of the body and the webbed 
feet, all clearly adaptations to an aquatic habitat. However, these adapta- 
tions do show that J'rionyr is in no sense directly ancestral to the other 
Testudinata; the Trionychia are to be regarded as an early offshoot of the 
main stem, which has retained certain of its primitive characters. 
State University of Oklahoma. 
Norman, Oklahoma. 
