'J7!S CAKHONIKEBOUS FORMATIONS AND KAUNAS OK (KtLOHADO. 



Till' only spciiiiioii ()))sci'V('(l in the Lcadvillc rcj^'ion consists of an impei-fct-t 

 external cast of a vontnil valve. The generic position of this .shell can not of comse he 

 oxuctly deterniined. Coniparint;- the surface ornamentation witli that of Dcrhya crama, 

 it IkS seen to be more linely striate; the strire ai-e not so strikingly alternating, and 

 seem not to be crossed by strong concentric orenulations. So far as these chai'acters 

 are exhibited, it agrees witli tlie shell from the Crested Butte region (station 2356) 

 identitied as Orthothetes InxqualiH, and it may readily belong to the same species. 



The conformation of the valve is similar to that of Drrhi/a //ashciHeihtM, but the 

 striation is finer and less alternating than in the prevailing form of the Chester 

 species. J can discover no intrinsic reason for holding it separate from the other 

 forms referred to OrtliiAlivtes inseQualis. 



The. only specimen representing this species from the Larimer County locality 

 (station 2364) is an impression of an internal cast of the dorsal valve. It can not, of 

 course, be determined whether this form is an Orthothetes or a Derhya. As far as 

 the material permits one to judge, it might be referred with almost equal propriety' 

 to either Ortliothetes insequalis or Derhya Icashashiensis. Derhya crassa, with which 

 White" identifies it, is distinguished from the Mississippian species just mentioned, 

 among other -diiierences, by the character of the concentric crenulations, which 

 are stronger and more crowded. This feature, however, is not preserved in the 

 impression before me, but there is a marked difference between the internal 

 structural characters shown by this impression and those possessed by the specimens 

 of D. crassa, which are illustrated by Meek'' and 113' White.'' It seems improbable, 

 therefore, that this impression is that of a specimen of Derhya crassa^ and as it does 

 , not differ materially from characteristic representatives of Orthothetes insegualis from 

 the Rico region and from Yellowstone National Park, I have referred it provisionally 

 to that species. 



Our material from Canj'on consists of two small internal casts, both probably of 

 dorsal valves. If a Derhya, they would probablj- be nearest related to the common 

 D. crassa, but the hinge plate, the impression of which is retained in the better 

 specimen, is less powerful and somewhat differently formed. In this particular it is 

 similar to the impression from Larimer County (station 236i), but it is more finel3' 

 striated and much smaller. In its size, which is but 18 mm. in transverse diameter, 

 shape, and surface characters, though imperfectly shown, this form rather strong!}^ 

 recalls Orthothetes lens, but here again the cardinal structures are in disagreement, 

 chiefly in the matter of the socket plates, which are nearlj^ jDai-allel to the cardinal 

 line in one case, but which in typical O. lens are directed to it at an angle of about 

 4z5^. On the whole it seems less likely that this form is a Derhya and related to D. 



f'U. S. Geol. Geog. Surv. Terr., Bull., vol. 5, 1879, p. 215. 



I>U. S. Geo! Surv. Nebraska, etc., 1872, pi. 5, fig. 106. 



I- Indiana, Dept. Geol. Nat. Hist., Thirteenth Kept., 1884, pi. 26, figs. 9, 11. 



