TRIONYCHIDS. 48J 



Family TRIONYCHIDS. 



Trionychoidea having the carapace devoid of peripheral bones, except in the case of 

 Trionyx {Emyda Gray)', in which the few nodules may be of secondary origin. Epiplastra 

 separated from the hvoplastra by lateral prolongations of the slender, archt or V-shaped ento- 

 plastron. More or less extensive fontanels between the hvoplastra, hypoplastra, and xiphi- 

 plastra of the two sides. Pelvis not suturally joined to either the carapace or the plastron. 

 So far as known, no epidermal scutes. Only 3 digits with claws. 



The Trionychidas constitute a family of tortoises remarkable on account of their structure, 

 their numbers, their geographical distribution, and their geological history. The oldest forms 

 at present known to us come from the Upper Cretaceous of New Jersey, Wyoming, Montana, 

 and Alberta, British America. It is probably not now possible to determine which deposits 

 are the older, the lower marl beds of New Jersey, which furnish the little known Amyda prisea 

 and A.? halophila, or the Judith River beds, from which we obtain Aspideretes coalescens, 

 A . foveatus, and A . splenduius. So tar as the shells are concerned, we find in the Judith River 

 forms onlv specific differences when compared with living species. Aspideretes beecheri, of the 

 Laramie and prohablv too of the Judith River beds, furnishes practically all portions of the 

 skeleton, except the skull; and these agree in all essential respects with the skeletons of living 

 trionychids. So far as the writer is aware, the oldest known trionychid skull is that of Con- 

 chochelys admtrabilis Hay of the Puerco deposits. This presents some differences when 

 compared with modern trionychids. The next oldest skull, that of Aspideretes singular! s, of the 

 Torrejon, differs little from that of Platypeltus ferox, now living in Florida. 



Trionychids are at present found in Asia, Africa, and North America. Asia furnishes at 

 least 15 species, Africa 5 species, and North America about 7. 



All these data point to a very ancient origin for the Trionychidae. We may expect to find 

 more primitive forms in fresh-water deposits of the Lower Cretaceous or even in those of the 

 Jurassic. It is probable that these primitive forms will approach more closely the Cryptodira 

 than do the Plastomenidae. 



A 1 . Skull broad, with narrowed pterygoids and wide maxillary alveolar surfaces Conchochelys 



I'. Skull unknown; carapace and plastron covered with pustular elevations Helopanoplia 



A 1 . Skull usually not so broad; pterygoid region of the palate broad: 



a 1 . Plastron smooth; no ridges and pits Axestemys 



a 2 . Carapace and plastron ornamented with ridges and pits. 



I. Eight pairs of costal bones; a preneural present Aspideretes 



z. Eight pairs of costal bones; no preneural Amyda 



3. Seven pairs of costals, or those of the eighth pair vestigial; no preneural. . Platypeltis 



Genus CONCHOCHELYS Hay. 



Skull broad. Masticatory surfaces broad and concave. Pterygoid region narrow. Basi- 

 occipital region short. Choana; behind the orbits. Posterior squamosal process apparently 

 much shortened. 



Type: Conchochelys admirabihs Hay. 



In its broadened form, its narrowed pterygoid legion, and in its apparently abbreviated 

 posterior squamosal processes, this trionychid presents such deviations from known genera that 

 the writer is led to the conclusion that it represents a genus distinct from all others hitherto 

 described. Until more shall have been discovered regarding the structure of the animal, the 

 characters given above may serve to distinguish this genus. The name Conchochelys, given 

 in reference to the supposed nature of the food of the turtle, was first publisht in the Bulletin 

 of the American Museum of Natural History, vol. xxi, 1905, page 3^5. 



Conchochelys admirabilis Hay. 

 Plate 88, figs. 1-3. 

 Conchochelys admirabilis, Hay, Bull. Amer. Mus. Nat. Hist, xxi, 1905, pp. 335-338, figs. 1-3. 



The skull which is here described was collected in the year 1892 by Mr. O. A. Peterson, 

 who accompanied Dr. |. L. Wormian, then in charge of a collecting party from the American 

 Museum of Natural History. It was obtained in the Puerco beds of the northern part of New 



