PLASTOMENID.*. 



477 



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 undescnbed 

 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. I; text-fig. 636) is 207 mm. along the midline. The 

 greatest width is 204 mm. There were no fontanels in the carapace, and the ribs extended 

 beyond 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; hence corresponding neurals 



636. 



637. 



FlGS. 636 AND 637. Plastomenus 

 visenJus. X J. 



636. Carapace. 637. Plastron. 



were not developt. The nuchal has a lateral extent of 90 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 5 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 hypoplastron are missing. The parts present are shown on the plate 



