46 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



Pecten (Delectopecten) arces Dall. Proc. Conch. Club So. Calif., 1944, 

 no. 35, p. 14.— Hertlein & Strong, 1946, p. 61. "Off Santa Cruz 

 Island, California, to Cedros Island, Lower California." 

 Pecten (Pseudainussiuin) tillamookensis Arnold. La Rocque, 1953 [ex 

 parte], p. 35. "A synonym of P. pedroanus (Trask) according to 

 Grant & Gale." 



Holotype: U.S. National Museum. 



Type locality: Off Tillamook Bay, Oregon; U.S. Fish Commission 

 station 3346, in 786 fathoms. 



Original description: Shell resembling P. randolphi in outline and 

 general characteristics. Disk ornamented by numerous more or less 

 prominent fine, radiating ridges, microscopic radiating striae, and incre- 

 mental lines of varying prominence. The radiating ridges are narrow 

 and thread-like, and appear to be more prominent anteriorly and near 

 the lines of interrupted growth ; the ribs are affected similarly to the 

 disk by the interruptions in growth. The microscopic striae cover the 

 whole surface of the disk and ribs and in a general way radiate from 

 the umbo, but are seldom parallel to the ribs ; on the portion of the disk 

 and ear posterior to the umbo the striae are perpendicular to the lines 

 of growth and are thus nearly parallel and not radiating. The right 

 anterior ear has from 6 to 12 prominently imbricated radials. 



Dimensions: Alt. 30 mm.; long. 30 mm.; hinge line 17 mm.; dia- 

 meter 7 mm. 



Additional description ; Disk circular in some specimens, higher than 

 long in others ; left valve more convex than right. Small and narrow 

 concentric undulations of disk often present either in umbonal region 

 only or recurring nearly to ventral margin. Radial ridges of disk promi- 

 nently spinose, continuing over both auricles of left valve and posterior 

 of right; "incremental lines" actually low concentric ridges covering 

 disk and less prominent than radials ; on some specimens both radial and 

 concentric ridges present only at margins. Anterior auricle of right valve 

 with usually 5 to 7 but occasionally as many as 12 imbricated radii; 

 wide fasciole, shallow byssal notch and ctenolium of 4 to 6 teeth. In- 

 terior surface of fold of posterior auricle of left valve angulate and 

 ridged. 



Remarks: The subspecies can be distinguished from the typical by 

 the sculpture of the disk, the latter being totally unsculptured except 

 for the anterior auricle of the right valve; the ridge inside of the pos- 

 terior auricle of the left valve also separates the subspecies. 



Geographical range: Bering Sea to Cedros Island, western Lower 

 California, Mexico. 



