22 



ALLAN HANCOCK PACIFIC EXPEDITIONS 



VOL. 23 



lower valve has, in the typical species, regular, thin, elevated, concentric 

 lamellae, which aid in the adaptation of the edge to that of the upper 

 valve; the margin is usually flattened or bevelled. The upper (left) 

 valve is radially sculptured, rarely smooth ; it usually has radial rows of 

 arched scales, pustules, or points, and also concentric raised lines; it is 

 sometimes cancellated. No radial ribs, nor interlocking points at the 

 margin. Auricles well developed, subequal, angulated and well-defined 

 at both ends ; byssal notch well defined ; few or no pectinidial teeth. 

 Cardinal folds single, rather feebly developed, often cross-lined. Eyes 

 few. Byssus small, and of few threads. 



Remarks: Some species do not have the ventral margin of the right 

 valve flexed, and one, Cyclopecten acutus sp. nov., does not have con- 

 centric lamellae or ridges on that valve. Most species are translucent 

 but a few are opaque. 



Cyclochlamys Finlay must be disregarded ; his diagnosis was based on 

 Pecten transcnna Suter, but is puzzling in that the criteria do not agree 

 with that species. Actually, P. transenna is clearly referable to Cyclo- 

 pecten. 



In addition to the genotype and the species included in this paper, 

 others referable to Cyclopecten are the eastern Atlantic imbrifer 

 (Loven) ; the western Atlantic simplex and leptaleus Verrill, subimbri- 

 fer and nanus Verrill & Bush, reticulus and strigillatus (Dall), and 

 culebrensis (E. A. Smith) ; the New Zealand transennus (Suter), 

 secundus Finlay and aupouria (Powell) ; the Kermadec Islands ker- 

 madecensis (E. A. Smith). 



Key to the eastern Pacific Species of Cyclopecten 



1. Shell very thin and translucent or subtranslucent ; right valve 

 concentrically ridged 

 2. Left valve with shallow concave radial fold next to posterior 



submargin cocosensis (Dall) 



(Cocos I., Costa Rica; Gulf of Panama) 

 2. Left valve with faint radial plications and radial rows of 



vesicles benthalis sp. nov. 



(southern California) 

 2. Left valve without fold or plications 



3. Surface smooth catalinensis (Willett) 



(southern California to Cedros I., Lower California; 

 northern Gulf of California) 



