120 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



concentric lamellae in juvenile stage, smooth when adult; anterior auricle 

 with 4 strong ridges covered with prominent concentric lamellae; 

 posterior auricle with 4 to 6 ridges crossed by fine lamellae ; byssal notch 

 small and ctenolium having 3 or 4 teeth. Left valve with 14 or 15 

 usually rounded but occasionally triangular ribs; interspaces slightly 

 wider and usually smooth, but occasionally having fine concentric lamel- 

 lae. Right valve usually predominantly white, with tops of ribs colored 

 in varying degree with darker or lighter shade of color of left valve. 

 Left valve very dark gray-black, slate, blue-gray, red-brown, pale brown 

 or orange, often mottled with white. 



Geographical range: Gulf of California to Paita, Peru. 



Geochronological range: Pleistocene; Recent. 



Bathy metric range: From just below low tide to about 70 fathoms. 



Ecological data: Usually found in mud, occasionally in sandy mud 

 or sand. 



Hancock Expeditions Collecting Stations: 

 ECUADOR: Manta Bay; 1 fathom, sand, sta. 402-35. 

 COSTA RICA: Salinas Bay; 8 fathoms, mud, sta. 476-35. 

 GUATEMALA: Ofi San Jose Point, 11 fathoms, black sand, sta. 770a- 

 38; off San Jose Light, 12-13 fathoms, fine black sand, sta. 930-39. 

 GULF OF CALIFORNIA: South of Consag Rock (31° N) ; 30 

 fathoms, mud, bottom sample 2119. 



Genus SEMIPALLIUM Jousseaume in Lamy 1928 



Semipallium Jousseaume in Lamy, 1928, p. 169. Type species (by mono- 

 typy) : Pecten tigris Lamarck, 1819, p. 171. Geographical range: 

 South Pacific, from Tuamotu Archipelago westward; western 

 Pacific, from New Guinea to southern Japan ; E^st Indies ; eastern 

 portion of Indian Ocean. 



Comptopallium Iredale, 1939, p. 359. Type species: Comptopallium 

 pauciplicatum Iredale, 1939, p. 360 [== Ostrea radula Linne, 1758, 

 p. 697]; type locality: Queensland. Geographical range: Indian 

 Ocean, from Mauritius eastward; Western and Northern Aus- 

 tralia, Queensland ; East Indies ; western Pacific, from New 

 Guinea to southern Japan. 



Complicachlamys Iredale, 1939, p. 362. Type species: Complicachlamys 

 wardtana Iredale, 1939, p. 362, pi. 5, figs. 25, 25a; type locality: 

 Queensland. 



Bractechlamys Iredale, 1939, p. 366. Type species: Bractechlamys evecta 



