PALEONTOLOGY ZORRITOS FORMATION 1 29 



ward; surface of valves evenly convex except posteriorly, 

 where a marked keel is bounded anteriorly by a broad, 

 shallow sulcus which grows narrower toward the beaks, 

 and posteriorly by a similar narrower concavity. In mature 

 specimens the keel is sulcate, having thus two edges, the sul- 

 cation flattening posteriorly and being stronger in the right 

 valve. The inner carinal ridge bounding the sulcus is often 

 the sharper of the two. Sculpture of concentric growth- 

 lines ; on the beaks are three to five sharp concentric waves. 

 Lunule deeply impressed, distinct, oval, bounded by a sharp 

 line, within which is another less intense line about one- 

 fifth of the distance toward the juncture of the valves; 

 escutcheon long and narrow. Hinge normal, the middle 

 cardinal tooth of the right valve sometimes pronounced. 

 Muscle scars deep; internal margins of valves smooth; 

 pallial line deeply incised. Length, 48 ; height, 32 ; diameter, 

 22.5 mm. 



Nelson's collection contains a group of Crassatellites, 

 labelled C. gibbosa Sowerby, which includes a few frag- 

 ments of the true gibbosa with a number of well-preserved 

 and very large specimens of C. nelsoni. A glance suffices to 

 distinguish nelsoni from the recent gibbosa; the former is 

 much more inflated, shorter, and the area is more deeply 

 impressed. The specimens of nelsoni in the Hopkins col- 

 lection are much smaller than those studied by Nelson, but, 

 as noted below, there is little doubt as to their identity. 

 Grzybowski described and figured the adolescent form as 

 Venus nelsoni. Comparison with his figure shows his form 

 to be identical with the specimens of C. nelsoni in the Hop- 

 kins collection, and although he did not figure or describe 

 the hinge beyond saying that it is "normal," there is small 

 doubt that his generic determination is at fault, and that his 

 Venus nelsoni is the Crassatellites here described. 



In the present description some distinction is made between 

 the characters of adolescent and mature forms. This is done 

 because of the apparent fact that at some localities the 



