140 CARBONIFEROUS PERIOD. 



Smface marked by from four to six strong, jDrominent, simple plica- 

 tions on each side of the mesial fold and sinus respectively". The whole 

 surface is also marked by tine granulations, and, near the front especially, 

 by concentric lines of growth, but no spines have yet been detected. 

 Exfoliated portions of the shell show its structure to be coarsely punctate. 



Length of a robust example, sixteen millimeters ; breadth, twenty-one 

 millimeters ; height, fourteen millimeters. 



This species, as represented in the collections, has all the characteristics 

 of the tyj^ical forms of S. octopUcata. S. Kentuchensis Shumard has l)een 

 by some authors referred to that species, but our shell is distinguished from 

 Shumard's species by its greater size, its more robust form, less numerous 

 and larger plications, and in the absence of the fine, prominent, concentric, 

 striation which that species possesses. It is very closely related to S. 

 sjnnosa Norwood and Pratten, from the Chester limestone of the Subcarbon- 

 iferous period at Chester, Illinois. Indeed, almost the only difference I am 

 able to detect between them is the apparently entire absence of spines from 

 the surface of our examples, which characterize that sj^ecies. Since we 

 often find among other species of spine-bearing Brachiopods that there is a 

 very great variation in the number of spines, even upon specimens associated 

 together in the same strata, it may not be unreasonable to suppose that 

 om- shell has lineally descended fi-om S. sjpinosa, suffering little or no change 

 other than the loss of its spines during the transition from one geological 

 period to the other. In my preliminary report, I regarded the examples 

 contained in the 'collections as those of a variety of S. spinosa, but even 

 that species is not unlikely a variety of 8. odoplicata. In any case, further 

 comjDarison has convinced me that our examples agree more nearly ■vidth the 

 last-named species than with 8. spinosa. 



This supposition of lineal descent seems to be supported by the fact 

 that more than one species found common in the Chester limestone forma- 

 tion is distinguishable from certain Coal-Measure fol'ms only with great dif- 

 ficulty, if at all. 



Position and localUy. — Strata of the Carboniferous period ; near Santa 

 F(^, New Mexico, and at Camp Cottonwood, Lincoln County, Nevada. 



