FAUNA OF THE EQUUS BEDS. 



77 



pared with T. liexagonata of tlie same horizon or locality, there are several 

 differences of importance, which are mentioned in the description of the 

 latter, which do not seem to be explicable on varietal or sexual grounds. 

 Eock Creek; W. F. Cummins (No. 46). 



TESTUDO HEXAGONAJA, Cope. - 



Sp. nov. Plate XXII, Figure 2. 



Represented by an entire chelonite; a considerable part of the plastron, 

 with fragments of costal bones of a second individual; and fragments of 

 plastron and carapace of a younger individual of probably the same 

 species. 



The chelonite is that of a species having much the form of the T. poly- 

 phemus and of tlie T. pertenuis of tlie Blanco bed. That is, it is a de- 

 pressed, rather short oval, witli wide extremities. The plastron is flat 

 and not sloped upward, as in T. polypJiemus, and the lip is relatively less 

 prominent than in that species. As compared with T. pertenuis the lip 

 of the plastron is less abrubtly contrasted with the rest of the anterior 

 lobe border, and the posterior lobe is less deeply emarginated. The size 

 is much less; but the value of this character is not now determinable. 



The lip of the plastron has a simple, regularly rounded outline, which 

 is distinctly set off from the rest of the anterior border. The thoracic 

 cavity excavates deeply its posterior border, so that the superior edge of 

 the latter overhangs it. This border soon fades out posteriorly, and 

 there is no angular ridge external to it posteriorly, as in T. laticaudata. 



The posterior lobe is rather deeply notched, and the angle bounding 

 the notch on each side is quite prominent; much more so than in T. 

 laticaudata, but not so much as in T. pertenuis. The external border is 

 distinctly notched at the femoro-anal suture, and is turned outwards 

 toward the apex of the posterior angle, which it is not in T. laticaudata. 

 The external ridge or border of the abdominal cavity reaches the median 

 suture at the fundus of the notch. 



The mesoplastral bone is a transverse hexagon, the lateral angles being 

 truncate. The borders so produced are shorter than the other four, 

 which are equal. The gulo-humeral scutal suture is straight and crosses 

 part of the mesoplastron. The humero-pectoral suture approaches, but 

 not very nearly, the pectoro-abdominal at the middle line. Tlie abdomino- 

 anal suture is, as in T. laticaudata, parallel in the corresponding border 

 of the xiphiplastral notch, and therefore forms an acute instead of a 

 right angle with the median suture. The posterior free marginal bones 

 are not stout. In one belonging to the presumed third specimen, the 

 supero-external surface is concave on each side of the scutal suture, and 

 the free border is entire. A part of the anterior border of the carapace 

 pot far from the middle line is slightly recurved at the edge, Below and 



