32 PALAEONTOLOGY OF CALIFORNIA. 



by numerous small concentric ribs, rarely dichotomous, abrupt 

 on the upper side, and sloping downwards on the side towards 

 the base. Muscular scars large, the posterior a third the largest, 

 broadly suboval, anterior triangular. 



Figure, natural size. 



Localities: From the Miocene, south of MartiSez, and north of Walnut Creek, 

 Contra Costa County, and a specimen in the collection of the Academy of Natu 

 ral Sciences, lahelled "Oregon," probably Astoria. This species has not been 

 found in the more recent Tertiary formation, though a fresh valve was picked up 

 by Dr. Cooper on the beach at Santa Cruz. In this specimen the epidermis is of 

 a bright olive-green and shining. 



The shell varies considerably in outline according to its age. In young shells 

 the beaks are about central ; but in the adults, the posterior end increases so much 

 more rapidly than the anterior, that the beaks are thrown forward so as to be 

 barely more than a third of the width from the anterior extremity. 



PECTEN, Brug. 



P. CERKOSENSIS, n. s. 



PI. Q, Fig. 55, 55 a. 



SHELL equivalve, subcircular, broader than long, convex; 

 beaks small; sides sloping concavely above, rounded below; 

 ears small, subequal, roughened and irregular, sinus very small. 

 Surface marked by eighteen or twenty flat ribs, with flat or 

 slightly concave interspaces; margins undulated, the ends of the 

 ribs being deeply emarginated, and the interspaces being pro 

 longed into tongue-like processes. 



Figure, one-half natural size (linear). 



LocalUy: Cerros Island, off the coast of Lower California: probably Miocene. 

 Collected by Dr. J. A. Veatch. 



P. YEATCHII, n. s. 



PI. 10, Fig. 56. 



SHELL large, equivalve, a very little broader than long; base 

 regularly rounded; sides sloping above, with a slight concavity; 



