144 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



equilateral and with smooth margins; base evenly rounded; sides slightly 

 concave above. Right valve somew^hat convex (more pronounced near 

 the umbo), and ornamented by about 22 or 23 prominent T-rail-shaped 

 ribs, flattened, and generally longitudinally ridged or sulcated on top, 

 which overhang deep, narrow, flat-bottomed interspaces; whole surface 

 sculptured by numerous fine, sharp, incremental lines; hinge line slightly 

 less than one-half length of disk; ears equal, slightly convex, rectangular- 

 ly truncated, sculptured by fine, sharp, incremental lines; anterior ear 

 with small byssal notch. Left valve flat or nearly so, generally concave 

 near apex, ornamented by 21 or 22 prominent, narrow, convex-topped 

 ribs, which are separated by concave-bottomed interspaces about equal in 

 width to the ribs ; whole surface prominently sculptured by numerous 

 regular, fine, sharp, concentric, raised lines ; ears concave, convexly 

 truncated, and ornamented by the same concentric sculpture as the disk. 

 Color, red and reddish brown, the left valve always being much darker 

 than the right. (Arnold, 1906, p. 128) 



Additional descriptive notes: The average adult specimen is 90 mm 

 in height, 110 in length and 20 in diameter. After his own description 

 of this species Arnold commented, "Mrs. Oldroyd has two specimens 

 taken from fishermen's nets at San Pedro, which measure over 150 milli- 

 meters (6 inches) in length, and are exceptionally high colored for this 

 species." 



Although Arnold said the right valve has 22 or 23 ribs and the left 

 21 or 22, such specimens must be quite uncommon; all that the author 

 has ever seen had 18 to 21 on the right valve and 17 to 20 on the left. 



The commensal gastropod Capulus cnliforniciis Dall is often attached 

 to the anterior side of the umbo of the right valve, as in Chlamys hnstata 

 (Sowerby), and lives only on these two species, never alone or on any 

 other mollusca. 



Remarks: Although closely related to Pecten sericeus Hinds, this 

 species is quite distinct and differs in the following respects: less orbi- 

 cular (length considerably greater than height) ; ribs of right valve 

 always squarish, flat-topped, and often longitudinally ridged ; ribs of 

 left valve wider and rounded ; interspaces of both valves narrower 

 (especially left) ; auricular margins perpendicular (or nearly so) rather 

 than oblique; hinge line relatively longer; concentric lamellae on disks 

 and auricles only half as numerous. 



Geographical range: Cordell Bank, of? Bodega Bay, northern Cali- 

 fornia, to Gorda Banks, off Cape San Lucas, Lower California. Also 

 recorded from Guadalupe Island, Mexico, 180 miles west of central 

 Lower California. * 



