PALEONTOLOGY ZORRITOS FORMATION 139 



vated, nearly central, curved inward and forward. Lunule 

 heart-shaped, very deeply impressed, two-thirds as wide as 

 long, marked by striations which become finer as they pass 

 into it. Anterior end short. Anterior and posterior ends 

 nearly equally rounded. Ligament large ; scar long, striated 

 longitudinally. Surface covered by a thick epidermis, and 

 marked by broad, flat concentric ribs, which become larger 

 and smoother over the middle of the shell, but not wholly 

 obsolete. With the epidermis removed the shell still shows 

 the striations, especially about the beaks. Hinge line nearly 

 straight, very broad. The median tooth (cardinal) of the 

 right valve is large and pointed ; the posterior cardinal deeply 

 bifid. Lateral tooth large, nearly as long as the posterior 

 cardinal, and parallel with it. In the left valve the median 

 cardinal is bifid throughout the upper half of its length. 

 Hinge area forming a very obtuse angle with the ligament 

 area. Muscular scars and pallial impression not observed. 

 A young and a full-grown specimen give the following meas- 

 urements : Young, length, 46; breadth (height) 47; height 

 (diameter) 22.6 mm. Mature, length, 95.6; breadth 

 (height) 95.2; height (diameter), 47.2 mm." Nelson, 1870. 



Nelson's description covers the characteristic features of 

 the species very well. It should be noted, however, that with 

 the epidermis removed the sculpture of the shell is quite 

 different ; there remains but small trace of the heavy concen- 

 tric waves which are a prominent feature of the unweathered 

 specimens, and the surface is crossed by fine concentric 

 growth-lines, some of which are heavier than others, and is 

 smooth with the exception of occasional shallow concentric 

 sulcae which are the only reminder of the pronounced undu- 

 latory sculpture of the epidermis. 



This large and characteristic species is represented in the 

 Hopkins collection by two individuals, the ventral margins 

 of which have been broken off, and the epidermis of which 

 has been eroded away with the exception of small patches, 

 but which are unmistakably identifiable with the abundant 



