TRIONYCHID. 545 
Platypeltis postera sp. nov. 
Plate 111, figs. 1, 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 somew hat above the level of the s specimens of P. he teroglypta, its 
nearest relative. The length of the carapace (plate 111, fig. 1) in a straight line is 315 mm.; 
the greatest width, 342 mm. The upper surtace 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. ‘he 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 g mm. 
| 
Neural. Length. | Width. = 
1 57 31 thick. These ends are abruptly beveled or truncated. 
: = The sculpture is better displayed than in P. heteroglypta, 
ener 3 | No. 5910, but has the same general character. It differs, how- 
| 5 ar : ever, in that the ridges between the pits are narrower and 
hl oa 16 e sharper. It consists of pits, which are very large on the neurals 
and the proximal ends of the costals, much sialic 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 fee are about 5 pits in the line indicated. 
The plastron (plate 111, fig. 2) lacks only Ae 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-ossifed with the hypoplastra. 
The lower surface of all these bones is covered with the sculptured layer. Ev erywhere, except 
on the outer ends of the hyoplastra and hy poplastra, the pits are nearly abvalete: On the outer 
ends of the bones mentioned there are about 5 pitsina 15mm. line. The line along which the 
hyoplastron and hypoplastron are united is 130 mm. The bridge is 64 mm. wide. ane 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 ian 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 Bridger 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 show! n 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 1s 
present nearly to the midline; and in case there were 8 pairs of costals the posterior must 
35 
