COMPLETE GRAU : PECTINIDAE OF THE EASTERN PACIFIC 99 



Chlamys (Plagioctenium) ventricosum (Sowerby). Bavay, 1936, p. 



315. 

 Pecten (Plagioctenium) circularis Sowerby. Hertlein & Strong, 1946, p. 



57. "Cedros Island, Lower California, and the Gulf of California, 



to Paita, Peru." 

 Aequipecten (Plagioctenium) circularis (Sowerby, 1835). Keen, 1958, 



p. 72, fig. 132. 

 Holotype: British Museum. 

 Type locality : Santa Elena, Ecuador. 



Original description: Pect. testa suborbiculari, tumida, subaequi- 

 valvi, aequilaterali, fusco alboque varia, auriculis magnis, subaequalibus ; 

 costis radiantibus octodecim interstitiis latioribus, arcuatim striatis; 

 valva altera sulcis profundioribus ; long. 1.5, lat. 0.8, alt. 1.4 poll. 



Additional description : Adult specimens averaging 65 mm in height 

 and 70 in length, but proportions variable; both valves very convex, 

 right slightly more than left. Umbos expanded, with beak of right valve 

 strongly produced above hinge line. Right valve with 17 to 21 squarish 

 ribs, flat and obscurely ridged on top, with a faint ridge on each side 

 just below the top ; interspaces narrow and produced beyond ventral 

 margin ; disk covered with very fine concentric lamellae, usually worn 

 off on adult shells. Anterior auricle shallowly convex and with 4 to 6 

 faint ridges; auricle and adjacent submargin having fine concentric 

 lamellae ; byssal notch rather deep and ctenolium of 3 teeth. Posterior 

 auricle shallowly convex, with 5 to 8 faint ridges; lamellae same as on 

 anterior auricle ; posterior sinus shallow. Left valve with narrower, 

 sloping ribs, occasionally grooved on top ; interspaces produced beyond 

 ventral margin ; disk covered with wavy concentric lamellae, more 

 prominent than on right valve. Anterior auricle moderately concave and 

 with 5 to 7 faint ridges; byssal sinus fairly deep; posterior auricle 

 shallowly concave and with 8 to 10 low, narrow ridges; both auricles 

 and adjacent submargins with fine concentric lamellae; posterior sinus 

 shallow. Hinge structure strong; cardinal crura very pronounced on 

 right valve and interlocking tightly with left valve; strong transverse 

 striae on crura and provinculum ; cardinal margin of right valve well 

 folded over; prominent nodules at base of each auricle on both valves. 

 Color range extensive ; left valve almost always darker than right. From 

 Gulf of California to Guatemala, various shades of red, pink, lavender, 

 brown, orange and yellow, usually mottled ; from Panama to Peru, 

 same colors plus black, purple-black, deep purple and violet, frequently 

 without mottling. Throughout range interior of right valve predominant- 



