86 SUUCAIJBONIFEEOUS PElilOD. . 



known fossil, -wliicli circumstance has naturally thrown nincli doubt on 

 the real specific identity of specimens collected from rocks of different 

 geological ages. 



It appears under slight varietal forms in* diffei'ent formations, which, 

 although not strongly marked, have served to induce some naturalists to 

 separate them as distinct species ; but after the scientific scrutiny these 

 shells have had for more than a hundi'ed years, no person has succeeded in 

 pointing out characteristics which entirely satisfy naturalists that they really 

 constitute more than one species. As thus specifically recognized, its range 

 is from the Trenton period of Lower Silurian age to the Subcarboniferous 

 period, and it is also a common fossil in both hemisperes. Therefore, 

 whenever this species is found associated with UpjDer Silurian or Devonian 

 fonns, and more especially if found unassociated with other fossils, it has 

 of itself no valuable significance as to the geological age of the strata in 

 which it is discovered; but when found associated Avith true Carboniferoiis 

 types, it is understood to indicate the /Sjt&carboniferous age of the strata 

 containing it, because that period is regarded as the extreme recent limit 

 of the range of the species in time. 



Position and locality. — Strata of the Subcai'boniferous period; Mountain 

 Spring, old Mormon road, Nevada, where it was found associated with the 

 Platycrinus and Actinocrinus herein described as w^ell as with other Subcar- 

 boniferous types. 



Family SPIRIFERID^. 



Genus SPIEIFER Sowerby, 1815. 

 Spirifer centronatus Wiiicliell. 

 Plate V, fig. 8 a, h, and c. 

 Spirifer centronata Winchell, 1805, Proc. Acad. Nat. Sci. Pliila., 118. 



Shell of medium size, iisually subsemicircular in outline, broadest at 

 the hinge-line; the cardinal extremities often cuspidate, but sometimes a 

 little rounded; both valves moderately convex. 



Dorsal valve broadly and regularly convex transversely, a little 

 depressed at the lateral extremities; convexity abrupt along the median 

 line fi-om the beak to the middle of the valve, but from the middle to the 

 front margin it is very slight or nearl}' straight; area very narrow; beak 



