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TRIONYCHIDA. 483 
Family TRIONYCHIDZ. 
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 hyoplastra by lateral prolongations of the slender, archt or V-shaped ento- 
plastron. More or less extensive fontanels between the hyoplastra, 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 Trionychidz 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, W yoming, Montana, 
and Alberta, British America. It is probably not now possible to determine w hich deposits 
are the older, the lower marl beds of New Jersey, which furnish the little known Amyda prisca 
and 4.? halophala, or the Judith River beds, from which we obtain As pideretes ‘coalescens, 
A, foveatus, and A. splendidus. So far as the shells are concerned, we find in the Judith River 
forms only specific differences when compared with living species. 4s prderetes beecheri, of the 
Laramie and probably too of the Judith River beds, filienighes 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- 
choc helys admurabilis Hay of the Puerco deposits. This presents some differences when 
compared with modern trionychids. The next oldest skull, that of Aspideretes singularis, 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 Trionychida. We may expect to find 
more primitive forms in fresh-water deposits “oe the Lower Cretaceous or even in those of the 
Jurassic. Itis probable that these primitive forms will approach more closely the Cryptodira 
than do the Plastomenide. 
A'. Skull broad, with narrowed pterygoids and wide maxillary alveolar surfaces...... Conchochelys 
A®. Skull unknown; carapace and plastron covered with pustular elevations. ...... Helopanoplia 
AS. sa usually not so broad; pterygoid region of the palate broad: 
. Plastron smooth; no ridges and pits......... Axestemys 
*. Carapace and plastron Sreamented with ridges and pits. 
1. Eight pairs of costal bones; a preneural present.... Re As pideretes 
2. Eight pairs of costal bones; no preneural ....... WP es Poe sate ial a Amyda 
3. Seven pairs of costals, or those of the eighth pair vestigial; no preneural. . Platypeltts 
Genus CONCHOCHELYS Hay. 
Skull broad. Masticatory surfaces broad and concave. Pterygoid region narrow. Basi- 
occipital region short. Choanae behind the orbits. Posterior squamosal process apparently 
much shortened. 
Type: Conchochelys admirabilis Hay. 
In its broadened form, its narrowed pterygoid region, and in its apparently abbreviated 
posterior squamosal processes, this trionychid presents auch deviations from known genera that 
the writer is led to the conclusion that it represents a genus distinct from all other 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 refenenee: to the supposed nature of the food of the turtle, was first publisht in the Ballin 
of the American Museum of Natural History, vol. xx1, 1905, page 335. 
Conchochelys admirabilis Hay. 
Plate 88, figs. 1-3. 
Conchochelys admirabilis, Hay, Bull. Amer. Mus. Nat. Hist. xx1, 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. J. L. Wortman, then in charge of a collecting party from the American 
Museum of Natural History. It was obtained in the Puerco beds " the northern part of New 
