TRIONYCHID. 529 
This species resembles in some respects . 1. francisce. However, the carapace of the latter 
is somewhat excavated at the midline in front, the rear is more truncated, and the ridges of 
the sculpture have broad and rounded summits. The anterior border of the hyoplastron i is 
very concave; that of 4. mura is gently sigmoid. Likewise, the mesial border of the hypo- 
plastron of A. francisce has a greater Admber of digitations. 
There are points of resenainaniee between 4. mira and Platypeltis he teroglypta. In the 
latter, as in others of the genus, the eighth costals are w anting or only feebly dey elopt. The 
form of P. heteroglypta is different from that of A. mira, the front of the carapace is more 
rounded and the rear more truncated. The nuchal is relatively longer and there is an 
unsculptured band along its front. It is observed, too, that the ribs on the underside of the 
costals are broader in P. heteroglypta than in the subject of the present description. The width 
of those of the second, third, and fourth costals of P. heteroglypta, taken near the distal end, is 
24mm.each. In 4. mira, the rib of the second is 22 mm. wide; the third, 20 mm.; the fourth, 
18 mm. P. heteroglypta was probably a flatter species, but the specimens are much crusht. 
Amyda? tritor Hay. 
Text-figs. 687-689. 
Aspidonectes tritor, Hay, Science (2), XIX, 1904, p. 254. 
Amyda? tritor, Hay, Amer. Geologist, xxxv, 1905, p. 336. 
This species is based on a large and well-preserved skull which was collected by the 
writer in the Bridger deposits, about 3 miles above the mouth of Cottonwood Creek, east of 
Fort Bridger, Wyoming. The matrix in which the specimen was found is a calcareous and 
argillaceous sandstone belonging to the base of the middle third of the Bridger beds. The 
number of the skull, in the American Museum catalog, is 5913. 
The total length of the skull (figs. 687-689) is 162 mm.; and it has therefore belonged to a 
large animal. In the American Museum of Natural History there is a specimen of Platy- 
peltis ferox, the carapace of which has a length of 265 mm. and whose skull has a length of 116 
mm. If the skull and carapace of 4. tritor had the same ratio that obtains in these parts in 
P. ferox, the carapace of the former would have had a length of about 370 mm. This is some- 
what less than the length of the carapace of Leidy’s Trionyx uintaénsis, and considerably less 
than that of the carapace of Cope’s 7. scutumantiquum. ‘The skull under consideration may 
therefore have belonged to one or the other of these or to some other described species. For 
the present it 1s necessary to give a distinct name to this skull and await further discoveries. 
The following are the principal measurements of the skull: 
Millimeters. 
Length from snout to end of supraoccipital spine 162 
Length from snout to end of occipital condyle 123 
Width from outside to outside of quadrates........ . 9I 
Width across pterygoids..... 47 
Width of interorbital space. . a : 20 
In general form the skull resembles that of Platypeltis ferox; but the width is somewhat 
greater, the snout is shorter and less pointed, the interorbital space 1s broader, and the choanz 
are more constricted. The facial angle is about the same in the fossil as in the living species, 
the roof of the mouth being bent own about 25° below the plane of the pteryg moire: The 
sutures between the various bones are more finely dentated than in P. ferox. The skull 
appears not to have suffered distortion during fossilization. 
The snout is broad and blunt. The vagal opening is 22 mm. wide and only 12 mm. high. 
The co-ossified premaxillz are small, measuring from side to side only g mm. The external 
surface of the maxilla is perpendicular, and the outline, seen from above, is slightly concave. 
Seen in profile, the skull is nearly flat behind the orbits, whilst more anteriorly there is a gradual 
curve to the end of the snout. In P. ferox there is an abrupt change of direction Reewe een the 
hinder borders of the orbits. 
The paroccipitals have about the same relative size and disposition as in P. jerox. The 
portion of the quadrate exposed between the prootic and the squamosal appears to be some- 
what narrower than in the living species mentioned, 
34 
