94 . 



This species is Iburid in .1 jjreeinsh-brown areniiceoiis rla\' deposit near the 

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

 tiie Fort Union or Lignite epoch. Collected by George M. Dawson, oC 

 Montreal, geologist of the British North American Boundary Commission, 

 near Woody Monnt. 



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 \\ell- 

 detined 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 rumiing 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. 



Measuremoits. 



M. 



'I'bickness of a costal .at the margin 0. 005 



Thickness of a costal at the rib 0.009 



Thickness of the free edge of the jilastron 0. 009 



'I'he costal bone of this species is much like that of a 7rioin/x, 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 F. coalescem, Comjjuemi/s ogmius, fragments of perhaps 

 Compsemys vidus and Trioni/x vagans, with Dmosauria, Cionodon stenopsis, 

 etc. (See chapter I.) 



I Plastomknus punctulatus. Cope. 



Established on a costal bone found in as.sociation with the preceding 

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

 ])ossibility that it will be found not to pertain to it when fully known. 



