142 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



less than half length of disk. Right valve gently convex; 22 or 23 ribs, 

 rounded in juvenile stage but usually sharply angled in adult; infre- 

 quently rather flat on top and with one or two grooves or a single ridge; 

 interspaces somewhat wider than ribs ; minute concentric lamellae cover- 

 ing disk, but often worn off tops of ribs; auricles equal, rather small for 

 size of disk, gently convex, with oblique margins, and covered with 

 minute concentric lamellae. Left valve nearly flat; central portion of 

 disk usually shallowly concave, but occasionally slightly convex; ribs 

 narrow and rounded in juvenile stage, usually becoming rather sharply 

 angled later; interspaces wide, very infrequently with a low riblet near 

 ventral margin ; auricles gently concave and with two or three weak 

 ridges; minute concentric lamellae covering disk and auricles. Right 

 valve white, yellow-white or pale brown ; left valve yellow-white, yellow, 

 orange-yellow, brown or red-brown, with both lighter and darker 

 blotches and streaks. 



Additional descriptive notes: Several adolescent specimens from 

 Cocos Island, Costa Rica, were examined and found to differ in being 

 more compressed, slightly higher than long (rather than the reverse) and 

 having a shorter hinge line. No doubt adult specimens are relatively the 

 same, but the author feels that the distinctions are not sufliciently impor- 

 tant to merit subspecific rank for this localized form. 



Geographical Range: Angel de la Guarda Island, Gulf of Cali- 

 fornia, to La Plata Island, Ecuador; Galapagos Islands. Previous 

 records indicated Santa Inez Bay, Gulf of California, to be the northern 

 limit, and Panama Bay the southern. Hancock expeditions collections 

 increased the northern limit by about 480 miles, the southern by about 

 500, and established the first Galapagos Islands records. 



Geochronological range: Known only from the Recent. 



Bathy?netric range: Recorded in 7 to 85 fathoms. 



Ecological data: Usually found in mud or sand bottoms, occasionally 

 rock ; associated with nullipores, calcareous algae, coralline and coral. 



Hancock Expeditions Collecting Stations: 



GALAPAGOS ISLANDS: Albemarle Island, 30 fathoms, rock, coral, 



nullipores, sta. 147-34; 58-60 fathoms, sand, nullipores, sta. 190- 



34; between Albany and James Islands, 50-70 fathoms, rock, sta. 



183-34. 



ECUADOR: Off La Plata Island, 7-10 fathoms, rock, nullipores, sta. 



213-34. 

 COCOS ISLAND, COSTA RICA: Chatham Bay, 47 fathoms, coarse 

 white sand, sta. 780b-38. 



