GEOLOGICAL SURVEY OF THE TERRITORIES. 617 



Trionyx concentetcus, Cope. 



Proceedings of the American Philosophical Society, 1872, p. 469 ; published July 29. 



This species reposes on various fragments, in one case representing 

 numerous portions of a carapace. The sculpture is intermediate between 

 those of T. heteroglyptus and T. guttatus. The costals have subequal 

 and subround pits throughout the entire length of the costal bones, but 

 their interspaces are raised into longitudinal ribs at intervals of from 

 one to three rows of pits. These ribs are equally developed at both 

 ends of the costals. 



Measurements. 



M. 



Width of a costal bone near the middle 0. 020 



Thickness of costal bone uear the middle 003 



The type specimen is smaller than that of the last. 

 From Cottonwood Creek. 



Trionyx guttatus, Leidy. 



Geological Survey of Montana, 1871, p. 370. 



!N'ot uncommon. 



Trionyx scutumantiquum, Cope, spec. nov. 



Established on a nearly perfect carapace and part of the plastrum 

 from the Bad Lands of Cottonwood Creek. These indicate the largest 

 species of the genus yet foand in North America. 



The carapace is a longitudinal oval, broadly rounded in front. The 

 median line forms a marked depression, and the costal bones rise and 

 descend again, forming an arch on each side. The free portion of the 

 ribs is not very long. The sculpture consists of numerous honeycomb- 

 like pits separated by rather narrow ridges. On the middle parts of the 

 carapace these are subequal, but on the middle of the length all the 

 ridges run together longitudinally, and ou their distal parts these are 

 broken up so as to produce innumerable irregular tubercles and pits. 

 The bones of the intercostal sutures are smooth. Eight costal bones, 

 the anterior co-osified with the second by its entire width, and sending 

 out a broad costal extremity which curves backward ; its anterior mar- 

 gin is smooth. Eight vertebrals, the last separating the anterior por- 

 tions of the last costals. 



Measurements. 



M. 



Length carapace 0. 425 



Greatest width carapace, axial 410 



Thickness of fifth costal 0075 



Thickness of fourth vertebra 034 



Thickness of centrum of fourth vertebra 010 



PLASTOMENUS, Cope. 



Allied to Trionyx. No (?) marginal bones of the carapace except a 

 nuchal ; extremities of ribs little, or not projecting beyond costal bones. 

 Plastron united with carapace by one or two tooth-like processes of the 

 hyosternal and hyposterual bones. An anterior production of the hyos- 



