CARBONIFERODS SPECIES. 73 



iiating at a spine-base, or tliree or four originating on the lower side, wliero 

 only one is continued above the same. The spines are large, erect, and 

 apparently long, and occur on the ears as well as over the other parts of the 

 valve, there being sometimes a row of them along the hinge-margin of each 

 ear. The dorsal valve is flattened, and generally provided with concentric 

 wrinkles on the visceral portion. 



Although this species is quite couunon, it is rarely found with the ears 

 and free margins unbroken. In most cases, only the umbonal and most gib- 

 bous parts of the ventral valve are preserved when the specimens ai-e broken 

 from a limestone matrix; while, owing to the thinness of the shell, specimens 

 in shale are usually crushed or much distorted. It seems to be much like 

 forms identified in' Europe with P. com, d'Orbigny, to Avhich it has been 

 by some referred in this country. It certainly differs from d'Orbigny's 

 figures of that species, however, in having its ventral valve more gibbous, 

 with more nearly rectangular and larger ears, with large strong wrinkles; 

 while its spines were certainly much larger and longer than repre- 

 sented by d'Orbigny. If his figure represents a specimen with tlie free 

 margins and the edges of the ears broken away, and the short, pointed spines 

 shown along the hinge are wrongly drawn, then his species may be identi- 

 cal Avith our shell ; but otherwise it seems to me to be quite distinct. Prof 

 De Koninck, however, who has seen d'Orbigny's original specimens, thinks 

 they are cei-tainly not distinct from the European forms referred to P. cora, 

 some of which are much more like our shell than the South American 

 tA'pical form would appear to be from d'Orbigny's figure. 



As in other species of the genus, the stria; of this shell vary somewhat 

 in size, though much less than is usual in most other species. On measure- 

 ment, I find that from three to five of them may be counted in a space of 

 one-tenth of an inch in the specimens under consideration. 



Localiti) ami position. — Fossil Hill, White Pine District ; Railroad 

 Canon, Diamond Mountains, Nevada: Carboniferous. 



