54^ 



FOSSIL TURTLES OF NORTH AMKRR'A. 



the midline is 132 mm.; the extreme width is 1 17 mm. The carapace (fig. 699) is moderately 

 convex. I he anterior holder is broadly rounded; the hinder border is excavated. There 

 were originally 6 neural bones, ot which the first is injured and the fourth is missing. The first 

 had a length of 21 mm. and a width of 10 mm. The others are shorter and narrower. The 

 nuchal has a fore-and-aft extent ot 18 mm. and a length of 66 mm. from side to side. There 

 are only 7 pairs of costals. Those of the seventh pair meet at the midline. The costals are 

 about 3 mm. in thickness. The sculpture of the specimen agrees with that ot the type, but it 

 is more feebly developt. The surfaces of the neurals and the proximal ends ot the costals 

 present shallow pits arranged irregularly. Of these pits there are from 5 to 7 in a line 10 mm. 

 long. Toward the distal ends of the costals the pits become smaller and more closely packt, 

 there being about 10 in a 10 mm. line. 



The greater portions of both hyoplastra and both hvpoplastra accompany the carapace. 

 There was a considerable fontanel between the hvpoplastrals in front and the xiphiplastrals 

 behind. The mesial half ot the hyoplastra and of the hypoplastic are only 3 mm. thick; but 

 the thickness is somewhat greater near the distal ends of the bones. The bridges are 26 mm. 

 wide. The length of the hvohypoplastral suture is 61 mm. The sculpture of these bones is 

 greatly like that of the carapace. 



Figs. 699 and 700. Platypeltis trionychoides. Xi. 



699. Carapace of No. 5<r,S A. M. N. H. 700. Plastron restored from No. 5938 and No. 6017 A. M. N. H. 



The limb bones present show no important differences when compared with those ot P. 

 spinifera. 



No. 6017, of the American Museum of Natural History, was found at Grizzly Buttes in 

 1903. It furnishes fragments of the costals. some neurals, and nearly the whole ot the plastron 

 except the entoplastron and the epiplastra, elements of the plastron rarely recovered, on 

 account of their loose attachment and their delicate construction. This specimen differs from 

 the preceding only in the boldness of the sculpture, the pits being deeper and the ridges higher 

 and sharper. It is coarsest on the outer ends of the hvoplastrals and the hvpoplastrals. From 

 this specimen has been drawn most of the plastron as represented in fig. 700, but parts of 

 this figure have been taken from the plastron of No. 5938. 



No. 5942, American Museum, also from Grizzly Buttes, belonged to a larger and an older 

 individual than the specimens already described. The hyoplastron is co-ossified with the 

 hvpoplastron. Following the line of the missing suture, the distance is 92 mm. The bones 

 are a little over 4 mm. thick on the mesial half; at the bridge they are about X mm. thick. 

 The sculpture is strongly exprest, especially on the bridges. 



The xiphiplastron differs in having the outer border, near the base of the process entering 

 the hvpoplastron, expanded outward. This is due probably to the greater age of the animal. 

 The bone is 5 mm. thick; that of No. 6017 is 4 mm. 



