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A Paired Entoplastron in Trionyx and its Significance.' 



By H. H. Lane. 



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 opinion 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 Trionychidse 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 Trionyx 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. 1 



