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



seems to lie in remarkable proximity to the maximum in a 

 curve plotted on this basis. 



The position of the mucro is very constant, but the degree 

 of elevation of the posterior area is extremely variable. The 

 accompanying series of figures (Plate XV, figs. 1-12) is de- 

 signed to illustrate this. 



When eroded to the proper degree, the radiating tubercles 

 of this species show each a distinct central pore or lumen. 



This is by all odds our most abundant fossil chiton, even 

 though this is entirely due to its enormous frequency in the 

 Upper San Pedro of Long Wharf Canyon, Santa Monica. An 

 amazing circumstance in this connection is the great prepon- 

 derance of head and tail valves among the specimens collected 

 (see foregoing table). Why only 17 out 197 specimens quar- 

 ried should be median valves, when the latter must originally 

 have been exactly three times as numerous as the terminal ones, 

 is not easy to understand. It is true that they are much less 

 heavy and solid, but as broken fragments are not more common 

 in the one case than the other, this does not seem an entirely 

 sufficient explanation. To a less degree the same phenomenon 

 is likewise true of C. crassico status in these beds. 



The largest specimens taken have the following caliper 

 measurements : 



BIBLIOGRAPHY. 

 Arnold, Ralph. 



:03. The paleontology and stratigraphy of the marine Plio- 

 cene and Pleistocene of San Pedro, California. Memoirs 

 California Academy of Sciences, vol. 3, p. 1-420, pi. 1-37, 

 June, 1903. 



:06. The Tertiary and Quaternary Pectens of California. 

 United States Geological Survey Professional Paper 47, p. 

 1-264, pi. 1-53, 1906. 



Ashley, G. H. 



'95. The Neocene stratigraphy of the Santa Cruz Moun- 

 tains of California. Proceedings California Academy Sciences, 



