1916] Moody: Fauna of the Fernando of Los Angeles 



57 



from the beak, and by about thirty-five prominent, regular, imbricating 

 lines of growth which curve outward, following the anterior margin ; 

 posterior margin makes a right angle with the hinge-line ; byssal notch 

 almost obsolete ; posterior ear ornamented with a similar number of 

 incremental laminae which are only slightly curved and are nearly 

 vertical, the radiating ridges being indistinct. Interior surface of the 

 valve marked by twenty-two rounding ridges which radiate from the 

 apex but are partially obscured in the upper portions by a callus 

 growth which covers one-third the area of the disc ; a single adductor 

 scar appears on the anterior dorsal margin of the callus. Left valve 

 slightly less convex than the right ; externally sculptured by fourteen 

 prominent, smooth, rounded ribs which radiate from the apex; in the 

 interspace appear one or sometimes two inter-ribs of varying length, 

 which never reach the beak ; anterior ribs and inter-ribs slightly imbri- 

 cated ; incremental lines quite fine and uniform on the posterior three- 

 fourths of the surface. Ears of the left valve subequal ; both slightly 

 arcuate below the hinge line, the anterior more coarsely sculptured 

 by twelve radiating striae and thirty or more roughened incremental 

 lines which are vertical on the hinge-line and broadly recurved below ; 

 beak sharp and straight and slightly raised above the hinge-line. 

 Interiorly the valve is sculptured by about twenty distinct, rounded 

 ribs radiating from the umbone, which are independent of the external 

 ornamentation and do not reach the ventral margin ; internal ribs 

 slightly obscured, as in the right valve, by a callus growth. 



Dimensions. — Latitude, 16 mm. ; longitudinal diameter, 15.6 mm. 



This little pecten is one of four members of the subgenus 

 Propeamusium known from the West Coast. P. davidsoni and P. 

 alaskensis are living forms now inhabiting deep waters from Van- 

 couver Island to Bering Sea ; the latter species is said to occur 

 as far south as Panama Bay, and is known from Pleistocene deposits 

 on Vancouver Island and in southern Alaska. Pecten riversi Arnold 

 is a closely related form from the Santa Monica Canon Pliocene. 

 Prom the descrijdion of P. riversi it appears that one of the chief 

 characters of the species is the retarded development of the internal 

 lirae, which do not appear until the shell has attained an altitude of 

 15 mm. In P. levis these internal riblets persist to the apex, a char- 

 acter of P. alaskensis. Other characters which serve to distinguish 

 this species from P. riversi are the smooth incremental lines on the 

 otherwise unadorned surface of the right valve, the lack of imbri- 

 cation on the ribs of the left valve, and in this valve, the failure of 



