PLASTOMENID&. 475 
The bones of the plastron which Professor Cope was disposed to take as the type of P. 
thomasi are now in the American Museum of Natural History, and are described under 
Platypeltis sertalts. 
Those specimens which Professor Cope figured in his great work of 1884 under the name 
P. multifoveatus must be regarded as the type of the present species. Of these specimens 
that which furnisht fig. 6 of plate xviii is 
now in the American Museum of Natural 
History. The others are in the U. S. 
National Museum at Washington. Of 
these figures, 2 and 5 represent the distal 
ends of 2 costals; fig. 3, the outer end of 
the right hyoplastron; fig. 4 is the left 
hypoplastron; and hg. 7, the right hypo- 
plastron. Figs. 2 and 5 seem to indicate 
that the sculpture is irregular, but in the 
text Cope states that the ribs intervening 
between the pits tend to connect into ridges 
running diagonally across the costal bones. 
These specimens were obtained on Cotton- 
wood Creek. 
The American Museum party of 1903 
operating in the Badlands of the Bridger 
Basin discovered a nearly complete shell 
which is identified as belonging to Cope’s 
P. thomas:. It was found at Grizzly 
Buttes, and the catalog number of the 
specimen is 6018. The carapace lacks only 
unimportant fragments. From the plas- 
tron there are missing the entoplastron, the epiplastra, the median end of the nght hyo- 
plastron, and the right xiphiplastron. 
The carapace (fig. 631) has a length of 218 mm. and a breadth of 210 mm. The upper 
surface was evidently moderately convex during life. The nuchal bone extends from side to 
side a distance of 97 mm. and from front to back 30 mm. There are present a preneural and 
six neurals. It is probable that there was a small seventh 
neural in a notch between the costals of the seventh pair. 
Fic. 631.—Plastomenus thomast. Carapace.  . 
No. 6018 A. M. N. H. 
Element. Length. | Width. The table shows the dimensions of these elements. 
The costals of the eighth pair meet along the midline 
Preneural.... 15 17 48 mm. and each measures 48 mm. along the free border. 
Neural 1 my, s) The outer ends of the ribs extend but little beyond the bor- 
Neural 2 20 14 ‘ i 5 
Neural 9). a 1 der of the disk. The outer ends of the costals are about 7 
Neural 4.. 18 14 mm. thick; and between the upper and lower layers of the 
Neural 5-4 175 na bone there is a channel running around most of the border. 
Neural 6.. 14 8 5 
On each side of the upper surface there are seen about 
8 low and broad welts; 2 or 3 of these extend on the nuchal. 
The nuchal and the outer ends of the costals are ornamented with shallow pits. These are 
mostly arranged in rows parallel with the border of the disk. A line 10 mm. long extends 
across 4, sometimes 5 pits. On the neurals and the proximal ends of the costals the pits are 
nearly obsolete. 
The form of the plastron and the proportions of its different parts may be seen in fig. 632. 
The width of the plastron between the ends of the hyohypoplastral suture is 210 mm.; the 
length along the midline, excluding the missing elements, is 135 mm. The bones of the 
opposite sides are joined by jagged sutures. The hyohypoplastral suture of each side makes 
an angle of about 75° with the median longitudinal suture. The greatest width of the hyo- 
plastron is 47 mm.; the width of the midline, 25 mm. The width of the hypoplastron at the 
midline is 49 mm.; of the xiphiplastron, 60 mm. The width of the hinder lobe, at its base, is 
100 mm.,; its length is 85 mm. The thickness of the hyoplastron at the midline is 5 mm.; 
