COMPLETE GRAU : PECTINIDAE OF THE EASTERN PACIFIC 131 



degree of prominence varying in individuals; interspaces with 6 to 8 

 radial ridges; entire disk profusely covered with fine imbricating lamel- 

 lae which is nearly always worn off tops of ribs on mature specimens. 

 Anterior auricle concentrically lamellose and with 10 to 15 radial ridges 

 of varying size; byssal sinus shallow. Posterior auricle same as that of 

 right valve. Hinge line with strong diagonal crura adjoining resilial pit. 

 Color range red-brown, red, red-orange, orange, yellow-orange or 

 yellow, with parts of shell white. Right valve usually predominantly 

 white, most of coloring occurring on ribs and auricles ; left valve of 

 darker and more evenly distributed color ; ridges of disk and auricles on 

 each valve either white or of paler color ; darker and/or lighter bands of 

 color on both valves. Interior red-brown, purple-brown or violet around 

 auricles and margins ; heavy white callus covering umbonal area, sur- 

 rounding muscle scar, and reaching to from one-third to two-thirds of 

 distance to ventral margin. 



Remarks: Specimens from western Lower California and the Gulf of 

 California to southern West Mexico are usually larger at maturitj^, 

 lighter in color, and with 10 ribs on the right valve and 9 on the left, 

 while those from central West IMexico (overlapping the above) to 

 Peru are usually smaller, darker, and with 1 1 ribs on the right valve 

 and 10 on the left. Conrad's Pecten inter medius was based on the north- 

 ern "form"; he commented, "It differs from subnodosus ... in having 

 much finer striae, numbering nearly double as many between each rib." 

 The finer striae are actually found only on occasional specimens, and the 

 features of the northern "form" mentioned above would not be of 

 specific or infraspecific significance even if they were constant. 



Variety P of Sowerby's Pecten ?nagnificus was almost certainly a 

 specimen of Nodipecten subnodosus ; its type locality, La Plata Island, 

 Ecuador, is the same, and only N. subnodosus has been subsequently 

 recorded from that area. Apparently A^ magnificus is restricted to the 

 Galapagos Islands, while N. subnodosus has not been found there. 



Of this species Dall said, "There seems to be little reason for 

 separating this form from the P. nodosus of the Antilles. Both vary 

 through a strictly analogous series of mutations." (1898, p. 710.) A 

 recent concurrence with that view was expressed by Dodge, who com- 

 mented, "It seems to be an ecological and evolutionary form of nodosus 

 and suggests a migration of that species in Miocene times by an ocean 

 passage through what is now Central America." (1952, p. 172.) The 

 important point in Ball's remarks is that the "series of mutations" by 

 which the species vary are "strictly analogous" ; an analogue, by defini- 



