154 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



(P. hancocki having 1 or 2 more on each valve), less complex surface 

 sculpture of the left valve, nearly smooth right valve and different 

 coloring. 



The holotype is the largest specimen found, but it does not seem to 

 be mature; the average size of other species in the subgenus would 

 indicate that its maximum height may be from 70 to 80 mm. 



This species is apparently restricted to the Galapagos Islands. Neither 

 Pecten vogdesi nor P. hancocki have been recorded from the area. 



Pecten (Oppenheimopecten) hancocki sp. nov. 



Plate 57 



Shell inequivalve and of moderate size, known specimens averaging 

 43 mm in height and 45 in length ; obliquity rather pronounced, with 

 disk produced posteriorly and posterior submargins longer and more 

 steeply angled than anterior ones; left valve smaller and recessed into 

 right valve at ventral margin to depth of 3 or 4 mm; hinge line less 

 than half as long as disk. Right valve quite convex, but less so than 

 Pecten vogdesi Arnold; umbo seldom produced above hinge margin; 16 

 or 17 strong rounded ribs, with a small riblet adjoining each submargin ; 

 interspaces narrower ; disk covered by very thin and concentrically striate 

 laj^er of shell, with fine concentric lamellae beneath ; lamellae visible 

 where outer layer worn off: on portions of submargins and in inter- 

 spaces of top one-third to two-thirds of disk. Interior of disk with 

 moderately deep grooves resulting from external ribs; reverse surfaces 

 of interspaces with edges angulated and occasionally thickened, but 

 usually not ; small elongate nodule at base of anterior auricle and larger 

 one at base of posterior; cardinal crura rather large in area but not 

 strongly produced. Anterior auricle rather small and shallowly convex ; 

 fasciole narrow, with concentric fold above it ; 2 or 3 stronger radial 

 riblets above fold and one or two very weak ones above them ; byssal 

 notch very shallow; ctenolium of 3 to 5 teeth present until shell reaches 

 height of about 25 mm, absent in adult specimens; surface of auricle 

 with same outer layer as disk and very fine (but seldom visible) con- 

 centric lamellae beneath. Posterior auricle with varying number of very 

 faint radiating ridges ; about 1 mm shorter than anterior auricle and 

 with same surface sculpture. Left valve almost flat, central portion being 

 only slightly depressed; 15 or 16 strong rounded ribs, with a low riblet 

 adjoining each submargin; interspaces a little wider; surface originally 

 covered by same thin striate layer as right valve, but outer layer worn 

 off of upper one-half to two-thirds of tops of ribs and almost completely 



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