44 PALAEONTOLOGY. 



the simis usually coalesce with the two marginal ones, Avhich also oenerally 

 each give off a lateral rib on the outer side; costse of the mesial fold more 

 or less bifurcating, while those of the lateral slopes of both valves are more 

 frequently simple, but sometimes divided. (Finer surface-markings un- 

 known.) 



Length, 0.63 inch; breadth, 1.18 inch; convexity, 0.57 inch. 



The type-specimens of this species were brought by Colonel Simpson's 

 jiarty, along with well-marked Devonian fossils, fi-om near White Pine, Ne- 

 vada. They evidently came from a dark, argillaceous limestone, exactly like 

 that from which the Devonian types were obtained; yet the species so nearly 

 i-esembles a form from a similar rock of the Carboniferous series farther 

 eastward, that I have sometimes feared that possibly these specimens might 

 have' been found in the latter,' and accidentally packed up along with the 

 Devonian specimens. This suggestion receives at least some support from 

 the fact that no such shell occurs among Mr. King's or any other collec- 

 tions I have yet seen from any of the Devonian rocks of the Far West. 

 The Carboniferous shell alluded to above, resembling this species, I have 

 been inclined to identify with S. Rocky-montana, of Marcou ; but still it seems 

 to shade off into forms even more nearly like S. ophnus, such as that repre- 

 sented by fig. 6 on our plate 9. 



On the other hand, however, there certainly are Devonian species very 

 closely resembling that here under consideration; such, for instance, as 

 S. Orestes of Hall and Whitfield, described from Devonian rocks in Iowa, 

 regarded by those authors as most probably of the age of the Chemung 

 Group of the New York series.* This Iowa shell so nearly resembles ours, 

 especially when ventral views are compared, that I was at first inclined to 

 believe them identical; but, on comparing the opposite view and profile, it will 

 be seen that S. Orestes has its ventral beak more prominent and less incurved, 

 and the cardinal area higher, with a broader fissure. Still shells much more 

 unlike are regarded by high European authoi'ities as being mere varieties of 

 one species in some cases. 



Locality and position. — Brought in along with Devonian fossils from a 



* For figure and descri[)tion of ti. Orcstc.si, sew :33d Report of the Kcgcnts of the 

 Uuiv. N. Y. on the State Cab. N. H., S.U, pi. 11, figs. 16-20. 



