432 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Description: Head valve evenly crescentic, the slopes only 

 slightly convex. Exterior somewhat worn and pitted ; surface 

 everywhere very finely and microscopically punctate, other- 

 wise practically smooth, without any well developed pattern 

 of sculpturing, except a few weak, concentric growth lines 

 (marked in the specimen at hand by faint, narrow, alternating 

 color bands of a darker tone than the body of the shell). In- 

 terior not examined, but the principal portion of the shell so 

 delaminated as to reveal the presence of a number of strong 

 radiating lines or grooves coincident with the marginal slits. 

 Slits and grooves about 13 in number. Insertion teeth smooth, 

 beveled somewhat at margin, but not very sharp. 



Median valve not beaked, relatively short and wide, mod- 

 erately high-arched; jugum with a strong but somewhat 

 rounded angle, the specimen being somewhat abraded in this 

 region; side slopes nearly straight. Tegmental surface un- 

 sculptured throughout except for a fine punctation and growth 

 striae like those of the head valve above described, the lines of 

 growth and concentric color bands here rather more conspicu- 

 ous ; lateral areas flattened, only weakly elevated and therefore 

 very indistinctly marked off from the pleural regions. Sutural 

 lamina? short and broad, their margins arcuate, with the inner 

 slopes more gradually tapering than the outer; well separated 

 in the median line. Interior little calloused, the radial grooves 

 on the sides 3-2; slits indistinct, but evidently 2-1, or 3-2, if 

 the posterior groove on each side be assumed to have originally 

 terminated in a slit. 



Tail valve rather triangular in outline, the mucro nearly 

 median ; depressed, the side slopes flat and straight, posterior 

 slope slightly concave; anterior margin strongly concave be- 

 tween the sutural lamina? ; boundary between central and pos- 

 terior areas clearly marked, forming an obtuse angle at the 

 mucro and thence extending straight to the sharp antero-lateral 

 angles of the tegmental margin. Sculpture exactly similar to 

 that of the median and head valves. Sutural lamina? excessively 

 short and broad, well separated. Interior with a strong, tri- 

 angular, median callus bearing numerous lateral branches. 

 Slits numerous but not everywhere definitely distinguishable 

 in specimen examined ; on the right side about 9 in number.' 

 Insertion teeth smooth, very short. 



' In the California Academy specimen the total number of radial lines can be 

 counted from above and appears to be 15. 



