470 FOSSIL TURTLES OF NORTH AMERICA. 
of the hyoplastron, 7 mm.; of the xiphiplastron, 5 mm. In front of the hyoplastra there was 
probably a small fontanel, indicated by a notch in each bone. The notch farther from the 
midline received one end of the entoplastron. The lower surface of all the plastral bones is 
covered with shallow pits. Most of these are in rows parallel with the free borders of the 
plastron. There are four, sometimes five, sometimes only three pits in a line 10 mm. long. 
Mr. Walter Granger, in 1903, discovered at Grizzly Buttes the skull and the outer end of 
the left hypoplastron of a trionychid which proves to belong to the present species. Judging 
from the portion of hypoplastron, the individual was about half-grown. The specific identity 
of this bone with that of the larger individuals is furnisht by a hypoplastron belonging to the 
Cope collection, No. 1030 of the American Museum of Natural History. Of the skull (figs. 
633-635) practically all portions are represented; but the palatal region has not been cleaned 
of the matrix. The premaxillary bone is wanting and the occipital condyle is broken away, 
but the length of the skull from the extremity of the one of these bones to the other has been 
close to 50 mm. From the snout to the end of the supraoccipital process has been 57 mm. 
The head is narrow, 24.5 mm. wide, slightly less than one-half the length to the occipital 
condyle. The snout is long and narrow. The upper surface of the skull forms a regular 
Fics. 632-635.—Plastomenus thomasi. Plastron and skull. 
3 aD 
632. Plastron. 4. No. 6018 A. M.N.H. 633. Skull, upper view. X1. No. 6015 A. M. N. H. 
634. Skull, side view. 1. No. 6015 A.M.N.H. 635. Lower jaw, from below. 1. No. 6015 A.M.N.H. 
curve from the snout to the end of the supraoccipital process. The orbit has an antero-posterior 
diameter of 10 mm., and the space between this and the nostril was the same. No postfrontal 
bone has been discerned. The postorbital process of the jugal reaches the parietal and this 
bone enters into the formation of the orbit, a very unusual arrangement. In Platypeltis 
spinifera the postfrontal bone is small, but it excludes the parietal from the orbit. In Plasto- 
menus thomas: the interorbital space has a width of 4 mm. The squamosal process was 
formed as in Platypeltis. The lower jaw (fig. 635) is narrowed in front; the symphysis is 14 
mm. long. 
The respects in which this species differs from P. visendus, referred in a former publication 
to P. thomas, will be given under P. visendus. 
Plastomenus visendus sp. nov. 
Plate 87, figs. 1, 2; text-figs, 636, 637. 
Plastomenus thomast1, Hay, Amer. Geologist, xxxv, 1906, p. 334, figure. 
Among the materials included in the Cope collection now in the American Museum of 
Natural History there was found a cigar-box of fragments which had been collected in the 
Bridger deposits of the Rattlesnake Hills, Wyoming, in 1895, by Mr. Stanley Stuart. On 
being fitted together these fragments furnisht a nearly complete carapace and plastron of a 
