94 



This species is found in a greenish-brown arenaceous clay deposit near the 

 Milk River in British America, belonging to the Transition series, probably 

 the Fort Union or Lignite epoch. Collected by George M. Dawson, of 

 Montreal, geologist of the British North American Boundary Commission, 

 near Woody Mount. 



Plastomenus costatus, Cope. 



Represented, in the collections made by Mr. Dawson, by small portions 

 of plastron and carapace, which display distinct osseous, but no dermal scutal 

 sutures. These specimens were discovered together, and are believed to 

 belong to the same individual. The bones are thinner than corresponding 

 ones of the two other species of tortoises described, from the same locality, 

 excepting at the costal enlargement, which is remarkably prominent and well- 

 defined on the under side of the carapace. The dense or cement layer of 

 the carapace is thrown into very delicate, but prominent ridges, which run 

 parallel to the axis of the carapace, and occasionally inosculate, or are crossed 

 by a similar ridge running at right angles to them. The sculpture of the plas- 

 tron is similar, but more obtuse and obscure. The superior edge of the free 

 border of one of the lobes of the plastron projects beyond the inferior, and 

 is not, as is usual, less prominent than the inferior. 



Measurements. 



ir. 



Thickness of a costal at the margin 0.005 



Thickness of a costal at the rib 0. 009 



Thick uess of the free edge of the plastron 0.009 



The costal bone of this species is much like that of a Trionyx, but the 

 character of the plastron refers it to Plastomenus. 



Collected in the bad lands of the Fort Union Cretaceous, south of 

 Woody Mount, near latitude 49°, British America. Associated with this 

 species were the P. coalescens, Compsemys ogmius, fragments of perhaps 

 Compsemys victus and Trionyx vagans, with Dinosaur ia, Cionodon stenopsis, 

 etc. (See chapter I.) 



? Plastomenus punctulatus, Cope. 



Established on a costal bone found in association with the preceding 

 species, and referred to the genus Plastomenus provisionally, and with a 

 possibility that it will be found not to pertain to it when fully known. 



