TRIONYCHID&. 539. 
growth than that of the specimen numbered 6014. The so-called callosities are much more 
extensive, a feature of age. Des have a width of about 40 mm. The length of the hyo- 
hypoplastral suture is 74 mm.; . the width of the bridge is 27 mm.; the thic kness at the bridge, 
7mm. A comparison of the femur (fig. 695) with corresponding parts of P. spinifera appears 
to show that the shaft was nowhere so slender as in the living species, while the distal end was 
not so broad. The parts of the pelvis present display no important differences when compared 
with the pelvis of P. spinifera. 
Mr. Paul Miller, of the American Museum of Natural History expedition of 1904, collected 
a nearly complete carapace (No. 3947) of a trionychid which the wniter refers to this species. 
It is shown on plate 109, fig. 1; text-fhg. 696. This came from the lower portion of the beds 
designated as.\Geandiis therefore of later date than the Grizzly Buttes specimens. The striking 
feature in the carapace is the feeble development of the longitudinal ridges, or welts, the 
specimen presenting thus a great contrast to No. 5944. The Tidges are moderately distinct 
on the last three pairs of costal plates, but farther forward they rapidly become obsolete. 
The pits are of the same size as in the other specimens and are separated by rounded walls. 
Fics. 694-696.—Platy peltis sertalis. 
694. Plastron. 3. 
695. Femur. X1. 
696. Carapace. <4. 
The nuchal of this specimen also is missing. It must have been very loosely attacht, 
just as it is in P. spinifera, even the first neural having its anterior border smooth. The left 
eighth costal is missing. The length of the specimen, as known, is 131 mm.; with the nuchal 
it was probably about 150 mm. The width of the disk, exclusive of the rib ends, is 105 mm.; 
but including these, it was about 175 mm. It must therefore have been a more elongated 
amimal than P. spinifera. The great extension of the rib-ends beyond the disk shows that it 
was only of medium age. 
The disk is rather fat, and was probably so during life. There are six neurals. The fifth 
comes into contact with only its own costals. The sixth is triangular and reduced in size. 
The costals of the eighth pair are small and hardly come into contact at the midline. 
In the Vertebrata of the Tertiary Formations of the West, 1884, p. 126, Professor Cope, 
in discussing the genus Plastomenus, writes as follows: ‘‘The true P. thomasi: was founded 
on sternal bone[s] perhaps of a small species of Trionyx.” Since in the earliest description of 
