UNDOUBTED TEHTIAllY Sl'EGlES. 183 



generally a little wider than the anterior, and faintly subtruncated, with an 

 anterior slope: dorsal outline rounding into the anterior and posterior mar- 

 gins, but moi'e regularly into the former. Beaks not eroded, neaily cen- 

 tral, rather prominent, and incurved, but not oblique. Surface ornamented 

 by sharply-defined, often elevated, concentric strite, separated by i-ounded 

 fun-ows, in which very minute lines of growth may be seen under a magni- 

 fier; the elevated concentric stride becoming more regular, coarser, more 

 distantly separated, and more prominent on the umbones. Cardinal margin 

 and lateral teeth comparatively stout. 



Length of largest specimen, 0.34 inch; height, 0.30 inch; convexity, 0.28 

 inch. 



TJie most mai-ked characteristics of this species are its quadrafo- 

 fiviborbicular, rather gibbous form, very nearly central beaks, and particu- 

 larly its sharply elevated, concentric strife, growing stronger, more 

 prominent, and more distantly separated on the umbones, until near the 

 points of the same they often assume the character of sharp, raised plica- 

 tions. In some of the smaller specimens, these raised, rather distantly 

 separated, stronger striae extend over nearly the whole surface; while in 

 others they pass gradually into mere irregular lines of growth on most of 

 the surface, occasionally sepai'ated by wider furrows. 



In form, this species is very similar to the existing S. Vcrniontanitm of 

 Prime, with which it also agrees nearly in size. It is more regularly 

 rounded in front, however, and has stouter lateral teeth; while its concen- 

 tric, raised striae and impressed sulcations ai-e generally lai-ger and grow 

 more distinct on the umbones than below, instead of the reverse. In this 

 latter character of marking, it agrees more nearly with .S'. <tiireiim, Prime, 

 from which, however, it differs entirely in form. 



Locality and position. — Fossil Hill, Kaw-soh Mountains, Nevada; in a 

 white calcareous matrix of Tertiary age. 



SPH^RIUM? iDAHOENSE, Meek. 

 Plate IG, tig. 1 aud 1 a. 



Sphcerhun? Idahoense, Meek (1870), Proceetl. Acad. Nat. Sci. Pliilad., Ap., 57. 



Shell attaining a large size, moderately convex, rather thick in propor- 

 tion to size, orbicular-subovate in outline, being wider in front than pos- 



