A Partrep EINTOPLASTRON IN TRIONYX AND ITS SIGNIFICANCE.” 
BY He A ann: 
There is no order of reptiles more distinctly circumscribed than the 
Testudinata. Even the fossil remains cast little if any light upon their 
affinities. That they are a highly specialized group need not be argued. 
Any point, therefore, which gives an indication of what may be considered 
to have been a primitive condition in the order, is of extreme interest and 
value. 
Moreover, there has been much discussion as to the relative rank of 
the various suborders and families comprised in this order. A group 
concerning which there is much diversity of opinieon is that now generally 
regarded as constituting a suborder, the Trionychia. Some have seen in 
their so-called ‘‘soft-shelled” condition, evidence of extreme specialization, 
and have therefore assigned them to a very high position in the order. 
Thus, Gadow (Cam. Nat. Hist., vol. viii, p. 406) asserts that “It is not 
open to much doubt that the characteristic features of the Trionychoidea 
are not primitive but secondary. This is indicated by the whole structure 
and behavior of the carapace and plastron. The softening of the whole 
shell, the loss of the horny shields, the reduction of the claws, are the 
direct and almost unavoidable results of life in muddy waters.” Other 
authorities take exactly the opposite view, and from the same facts reach 
the conclusion that “the Tricnychidze stand nearest to the general struc- 
tural plan of the Reptilia” (Adolph Th. Stoffert, Structure and Develop- 
ment of the Shell of Emyda ceylonensis, Gray). 
On account of this difference of opinion the writer has undertaken a 
study of the embryonic development of Trionyxr with the view, first, of 
determining, if possible, the relative position of the Trionychia among the 
Testudinata, and, second, if it should prove to be a comparatively general- 
ized type, to secure some hint as to the reptilian form from which the 
chelonian ancestry may have been derived. I present in this paper only 
one phase of the evidence furnished by the plastron, relative to the first of 
these two problems, although my material sheds some light upon both. 
* (Contribution No. 5, from the Department of Zoology and Embryology, State 
University of Oklahoma.) 
