162 PALAEONTOLOGY OF CALIFORNIA. j 



These two forms are the most common, and I have before me every possible grada- 

 tion between them. The form, figure 141, is more rare, and seems to be confined 

 to the northern portions of the State. Specimens from Pence's and from Tus- 

 can Springs, Tehama County, usually approach it more or less closely. Although 

 the shell represented in figure 141 appears to be widely distinct in outline, shape 

 of the umbones, &c, from the others, still, after studying a series of not less than 

 a hundred specimens, I can find no good grounds on which to separate it. 



On account of the nature of the matrix, I have been unable to examine all tin- 

 details of the hinge, and the generic determination must therefore remain, tu u 

 certain extent, in doubt, for the present. 



V. Veatchii, n. s. 



PI. 23, Fig. 142. 



Shell small, subquadrate ; beaks large, anterior, nearly termi- 

 nal ; cardinal margin sloping slightly convexly to the posterior 

 end, which is rounded-subtrnncate ; anterior slightly concave 

 above, narrow : convex below ; basal margin nearly straight ; a 

 broad, rounded ridge passes from the beaks to the posterior basal 

 angle, in the rear of which the surface is nearly flat. 



Surface marked by moderately large, regular, concentric ribs. 



Figure, magnified from three to four times. 



Locality: Abundant in Division A., at Tuscan Springs; collected by Dr. Veatch. 



V. LENTICULARIS, 11. S. 

 PI. 30, Fig. 246. 



Shell thick, subtriangular, compressed; beaks high, a little 

 in advance of the centre; posterior cardinal margin sloping 

 very abruptly, slightly convex; anterior side broadly excavated 

 under the beaks, convex below ; base broadly and regularly con- 

 vex. Lunule small, rather distinct. Internal margin not crenu- 

 lated. Surface marked by regular, rounded, concentric ribs. 



Figure, natural size. 



Locality: Benicia ; collection of Mr. Eemond. 



