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



dorsal ridge, in the more numerous lirae of the central areas 

 (40-45 in heathi), in the more definite radial arrangement of 

 the granules on the lateral areas, and in the conspicuously 

 shorter and less pointed sutural laminae. 



From L. rugatus (Carpenter) it is separated by the weaker 

 growth lines and consequently less conspicuous terracing of the 

 lateral areas, together with the less crowded and more dis- 

 tinctly radial arrangement of the granules in this region, the 

 more angular ridge, and the shape and position of the sutural 

 laminae. 



Fig. 7 



Fig. 7. Leptochiton rugatus (Carpenter). Dorsal aspect of right side 

 of valve iv of recent specimen from La Jolla, California, [Ilia]; same 

 scale ; camera outline. 



Fig. 8 



Fig. 8. Leptochiton rugatus (Carpenter). Ventral aspect of same; 

 same scale ; camera outline. 



The fossil species differs from both L. rugatus and L. heathi 

 in the very short and wide sutural laminae, which in L. rugatus, 

 moreover, are not marginal, but abruptly set over and separated 

 by a sharp notch from the lateral margin of the shell. The 

 differences in form of the sutural laminae in these three species 

 are very clearly brought out in the accompanying camera draw- 

 ings (text figs. 3-8). L. nexus I have unfortunately not had 

 opportunity to disarticulate. 



The specific name is chosen in honor of Dr. F. C. Clark of 

 Santa Monica, California. 



