UNDOUBTED TERTIARY SPECIES. 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 more regularly into the former. Beaks not eroded, nearly cen- 

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

 by sharply-defined, often elevated, concentric strise, separated by rounded 

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

 fier; the elevated concentric strise 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. 



Tlie most marked characteristics of this species are its quadrato- 

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

 larly its sharply elevated, concentric strise, 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 strise extend over nearly the whole surface; while in 

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

 the surface, occasionally separated by wider furrows. 



In form, this species is very similar to the existing S. Vermontammi 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 strise and impressed sulcations are generally larger 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. aureum^ 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. 



Sphjerium? Idahoense, Meek. 



Plate 16, fig. 1 and 1 a. 



Splicerium? Idahoense, Meek (1870), Proceed. Acatl. Nat. Sci. Philad., 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- 



