96 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



Plagioctenium was proposed by Dall to separate from Aequipecten 

 those species which conform to his diagnosis, but he was apparently un- 

 aware that Monterosato had created a supraspecific unit for them nine 

 years earlier. 



Coryfnbichlamys Iredale (another of his monospecific genera) was 

 based on the lattice-like ornament between the ribs and the lamellae and 

 small nodules on the ribs of the type-species. In all other respects that 

 species is referable to Argopecten, and the present author does not regard 

 its sculpture as having significance above specific level. 



In addition to the type-species, the following are also referable to 

 Argopecten: the west African fiabella (Gmelin) ; the western Atlantic 

 gibba (Linne), gibba nuclea (Born), gibba portusrcgii (Grau), noron- 

 hense (E. A. Smith), irradians (Lamarck), irradians concentrica (Say), 

 irradians amplicostata (Dall) and fiabella schra?nfni (Fischer) ; the 

 eastern Pacific circularis (Sowerby), circularis aequisulcata (Carpenter) 

 and purpurata (Lamarck) ; the central Pacific, western Pacific and 

 Indian Ocean nux (Reeve) ; the western Pacific (Japan, Philippine 

 Islands and Siam) pelseneeri (Dautzenberg & Bavay) ; the Queens- 

 land, Australia corymbiata (Hedley). 



Key to the eastern Pacific species of Argopecten 



1. Shell quite convex and umbos strongly inflated 



2. Shell rather thick ; left valve almost as convex as right ; colors : 



black, brown, red, purple, lavender, pink, orange, yellow; 



adult shells averaging 66 mm in height 



circularis (Sowerby) 



(Cedros Island, Lower California to Paita, Peru; Galapagos 



Islands) 

 2. Shell thinner ; left valve much less convex than right ; colors : 



red-brown, dark brown, mottled with white or yellow ; adult 



shells averaging 80 mm in height 



circularis aequisulcata (Carpenter) 



(Santa Barbara, California to Bahia San Quintin, Lower 



California) 

 L Shell less convex and umbos less inflated; ribs profusely lamellose 

 along sides; interior with broad, flat ridge on lower ^ of disk 

 and hinge line black; colors: usually white with purple ribs, 

 occasionally lavender or yellow ribs, sometimes mottled orange, 

 white and brown, rarely all white 



