PLASTOMENID. 47 
species of Plastomenus. It was originally supposed that this was Cope’s P. thomasi, but the 
fortunate discovery of a specimen of the latter at Grizzly Buttes, near the type locality, shows 
that the specimen from the Rattlesnake Hills belongs to a distinct and hitherto undescribed 
species. The catalog number of the specimen is 1895. 
In form this species is broadly oval, with the broadest portion somewhat in front of the 
middle of the carapace. The latter appears to have been only moderately archt. The total 
length of the carapace (plate 87, fig. 1; text-fg. 636) is 207 mm. along the midline. The 
greatest width is 204 mm. ‘There were no fontanels in the carapace, and the ribs extended 
bey ond the disk probably nowhere more than about 12 mm. There is no evidence that there 
were any peripheral bones. The nuchal is firmly united with the first pair of costals. Only a 
fragment of the preneural is preserved. There are present remains of 4 neurals. Of the first 
there is only a fragment. The fifth and sixth neurals are missing. The costals of the seventh 
and eighth pairs meet in the midline for their whole breadth; ine corresponding neurals 
—— 
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Fics. 636 AND 637.—Plastomenus 
vise re «4. 
636. Carapace. 637. Plastron. 
were not developt. The nuchal has a lateral extent of go mm.; and a fore-and-aft extent of 
20 mm. Its outer ends neither overlap nor underlap the anterior border of the first costals. 
These costals are somewhat narrower at their distal ends than at the proximal. The other 
costals, except the seventh, are wider at the outer ends than proximally. The seventh costals 
are much wider along the midline than any of the others, and form a large part of the rear of 
the shell. They extend along the midline a distance of 50 mm., and laterally from the midline 
a distance of 75 mm. The edges of the carapace are rounded or cut off rather abruptly. The 
thickness of the costals at the free edges is about § mm. The only neural which is complete 
is the third. It has a length of 18 mm. and a width of 12 mm. posteriorly. 
The ornamentation consists of a number of longitudinal welts and of numerous pits. Of 
the welts there are about 10 on each side. These resemble a series of waves, those near the 
midline being somewhat nearer together than those near the borders of the carapace. On the 
hindermost costals the welts are sharper and somewhat broken up into elongated nodules. 
On the nuchal and costals of the first pair the welts are obsolete. The pits are rather shallow 
and are separated by ridges which are rounded or flat on their summits. There are 5 or 6 of 
the pits in a line 10 mm. long. They are less distinct on the neurals and the proximal ends of 
the costals. On the outer halves of most of the costals the pits are arranged to a great extent 
in rows, parallel with the axis of the animal or with the adjacent free border of the carapace. 
Of the plastron (plate 87, fig. 2; text- fig. 637) the entoplastron, both epiplastra, and a 
small portion of the left hy poplastron are missing. The parts present are shown on the plate 
