TRIONYCHID^. 



4 8 9 



This species was based on a specimen which was collected in the basin of Milk River, 

 south of Wood Mountain, Assiniboia, British America, and which is now in the collection of 

 the Geological Survey of Canada. Professor Cope's earliest mention of the name was not 

 accompanied by a description; but in the same year both description and figures were publisht. 

 The figures represent the bones half the natural size. 



Professor Cope thought that there had been secured portions of both the carapace and 

 plastron of this species; but an examination shows that all the parts belong to the plastron. 

 The bones which furnisht his fig. 6 and 6a (Vert. Cret. Form. West, plate viii), instead of being 

 parts of costal plates, are the median ends of the right hyoplastron and hypoplastron; while 

 the original of fig. 7 is made up of other parts of the same bones. In the figure here presented 

 (fig. 645) that portion on the left of the curved line running from A to B is the part represented 

 by Cope's fig. 7, the lower end of the latter figure corresponding to the left side of our fig. 645. 

 The part of the latter figure which lies on the right of the line from A to C represents the same 

 mass of bone as Cope's fig. 6. The portion of our fig. 645 lying in the triangle ABC, except 



the part dotted, represents a frag- 

 ment of bone which was present with 

 the others, but which Cope did not 

 bring into relation with the rest of 

 the plastron. This comes into accu- 

 rate contact with the other two 

 pieces and connects them. 



Professor Cope thought too that 

 the sutures between the various 

 bones had been obliterated, but one 

 may be traced from end to end of 

 the united fragments. In Cope's 

 fig. 7 the suture would run perpen- 

 dicularly thru the middle thereof; 

 while in fig. 6 it would start 23 mm. 

 below the sharp projection to the 

 right of the deep notch on the right 

 side and cross the figure to a point 

 on the upper margin 37 mm. from 

 the extreme left angle of the figure. 

 We have in this specimen, there- 

 fore, a large part of the hyoplastron 

 and hypoplastron of the right side, 

 and belonging to a very large animal. It is quite certain that the remains did not belong to 

 the genus Plastomenus. 



The width of the plastron, where narrowest, between the axillary and inguinal notches, 

 is nearly 70 mm., and the thickness is 18 mm. Toward the median portion of the bones the 

 thickness is reduced to 10 mm. or less. The process of the hypoplastron which is nearest the 

 hyohypoplastral suture and directed toward the midline is very short, leading to the opinion 

 that the fontanel between the right and left halves of the plastron was narrow. 



The ornamentation of the plastral bones which form the type of this species is mostly 

 quite obscure. On the portion of the plastron forming the bridge there are rather coarse 

 ridges forming by their union large pits. Toward the median portion of the plastron the 

 sculpture becomes mostly effaced. 



Mr. L. M. Lambe, of the Canadian Geological Survey, has described, under the name 

 Trionyx vagans, a large specimen of a trionychid, which he secured in the Belly River deposits, 

 in the region of the Red Deer River, below Berry Creek, in Alberta. The present writer, 

 regarding Cope's type of Trionyx vagans as too small and imperfect a fragment for satis- 

 factory comparison with materials from any region, except the type locality in eastern Colorado, 

 is compelled to seek for some more probable disposition of Mr. Lambe's fine specimen. Since 

 the plastron of Cope's Plastomenus coalescens indicates a large trionychid which lived in 

 approximately the same region and in the same geological period, it appears to be best to 

 refer the Red Deer River carapace to the same species. 



FIG. 645. Aspideretes coalescens. Plastron of type. X J- 

 A, B, C, explained in teit; hyo, hyoplastron; hypo, hypoplastron. 



