TRIONYCHID^. 



545 



Platypeltis postera sp. nov. 

 Plate ill, figs. I, 2. 



The type of this species is No. 6133 of the American Museum. It was collected by the 

 museum's expedition of 1905, at Sage Creek Springs, Wyoming, in deposits belonging to the 

 level known as C, therefore somewhat above the level of the specimens of P. heteroglypta, its 

 nearest relative. The length of the carapace (plate in, fig. i) in a straight line is 315 mm.; 

 the greatest width, 342 mm. The upper surface is strongly convex from front to the rear and 

 from side to side. This form is certainly natural. The nuchal- is 39 mm. from front to rear, 

 182 mm. from side to side. There is no smooth band along its front. The 7 neurals are all 

 present, and have the dimensions shown in the table. The seventh partly separates the 

 costals of the seventh pair. Behind this these costals join along the midline a distance of 57mm. 



The left one of the pair measures along the free hinder border 80 

 mm. The costals are thicker than in either of the specimens of 

 P. heteroglypta but this may be due to its being an older indi- 

 vidual. At their distal ends and near the suture they are 9 mm. 

 thick. These ends are abruptly beveled or truncated. 



The sculpture is better displayed than in P. heteroglypta, 

 No. 5910, but has the same general character. It differs, how- 

 ever, in that the ridges between the pits are narrower and 

 sharper. It consists of pits, which are very large on the neurals 

 and the proximal ends of the costals, much smaller on the distal 

 ends of the costals and on the front of the nuchal. In the median 

 region there are 3 pits in a line 13 mm. long. Occasionally they are slightly larger than this. 

 On the distal ends of the costals there are about 5 pits in the line indicated. 



The plastron (plate in, fig. 2) lacks only the entoplastron, the epiplastra, and the outer 

 ends of the right hyoplastron and hypoplastron. The form and relative size of the various bones 

 may be determined from the plate. The hyoplastra are solidly co-ossified with the hypoplastra. 

 The lower surface of all these bones is covered with the sculptured layer. Everywhere, except 

 on the outer ends of the hyoplastra and hypoplastra, the pits are nearly obsolete. On the outer 

 ends of the bones mentioned there are about 5 pits in a 15 mm. line. The line along which the 

 hyoplastron and hypoplastron are united is 180 mm. The bridge is 64 mm. wide. The bones 

 are about 12 mm. thick. 



A specimen, No. 5955 of the American Museum, was collected in 1906 at Grizzly Buttes, 

 Wyoming. It is smaller than the type; but the nuchal resembles that of the latter, having the 

 sculptured layer covering the whole upper surface. 



This species is certainly closely related to P. heteroglypta. The shell was probably more 

 convex in life than that of the latter species. The bones are thicker and the free borders are 

 more abruptly cut ofF than in P. heteroglypta. The nuchal is longer from side to side and 

 there is no smooth band along the free border. Nor is there a smooth band of any considerable 

 width on the free ends of the costals, as there is in both specimens of P. heteroglypta. 



Platypeltis extensa sp. nov. 



Plate 112; text-fig. 703. 



The species here described is represented by only an imperfect carapace and plastron 

 which were secured by the writer during the summer of 1903, in the Bndger deposits of the 

 western part of Grizzly Buttes, Wyoming. When discovered the specimen was much broken. 

 The fragments have been fitted together with all possible care, with the result that the form 

 and the essential structure of the shell are revealed. A considerable area near the front of 

 the carapace is yet missing; likewise a portion of the rear. The position and extent of the 

 missing portions are shown by the figure on plate 112. The catalog number of the type is 

 5951 of the American Museum of Natural History. 



In form the carapace is broader than long, with a broad and somewhat rounded front and 

 a broadly truncated hinder end. The upper surface possest a moderate convexity, and this 

 is probably not different from the form in life. The total length is 300 mm.; the width, 345 

 mm. Apparently there were present only 7 pairs of costal plates. Posteriorly the bone is 

 present nearly to the midline; and in case there were 8 pairs of costals the posterior must 



35 



