114 THE WASATCH AND BRIDGEK FAUNiE. 



remains obscure, the following observations may be derived from the study 

 of the forms in question. 



The order makes its appearance in the Triassic period, for I am assured 

 by Dr. F. Endlich, of Reading, Pa., that the species obtained by Professor 

 Quenstedt, in Wiirtemburg, belong undoubtedly to the Testudinata. With 

 their special structure we are not yet fully acquainted. A number of genera 

 appear in the Jurassic, and there is a successive increase in the number of 

 species in the Cretaceous and Tertiary formations. Sphargis, which is with- 

 out carapace, and has a greatly reduced plastron, may be regarded as 

 nearest the primitive types of the order. One species, S. pseud ost radon, 

 has been found in the European Jurassic, and the genus exists in recent 

 seas. Protostcga of the Kansas Cretaceous is its nearest extinct ally known. 

 Protostega is superior in the well- developed marginal bones, and prepares 

 the way for consideration of the genera with incomplete shields, of the pres- 

 ent period {Cheloniidce), which also possesses the natatory extremities. Two 

 structural features of importance mark many of the Jurassic forms. First, 

 the incomplete union and ossification of the elements of the plastron and 

 carapace; second, the reduction in size of the lobes of the plastron. Genera 

 retaining some or all of these peculiarities persist to the present day, but 

 the ossified types with wide plastron are far more abundant, and are com- 

 paratively rare in the period of the Jura. 



Trionyx appears to represent another point of departure. Its plastron 

 presents a grade of development near to that of Propleura, and its eight 

 costal bones ally it to other types. In its half-ossified carapace wanting 

 the marginals, it is inferior to both. It leads us at once to the existing Chcly- 

 dra, the closing of the sternal fontanelles being accompanied by a contrac- 

 tion of its extent in respect to the bridges and lobes. In Propleura of the 

 Cretaceous we have a state of things intermediate between some of the 

 Jurassic Chelydridcc, as Idioclielys, and Chelone. These genera had a common 

 origin near the Jurassic predecessor oi Protostega. The peculiar sculpture of 

 Trionyx IS seen in the Eocene Anostira (which is much like CJielydra in form), 

 and in an obsolete condition, in Adocus of the Cretaceous, which adds 

 Chelydrino and Pleurodire characters in a remarkable manner. It is closely 

 joined by Pladomenus, which, in turn, presents points of resemblance to 

 Emydidw. 



