106 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



appear in reverse on the interior surface. The internal ribs are not 

 angulated by a deposit of shell, nor distinctly thickened. Margin with 

 broad scallops. The exterior surface is covered w^ith fine divergent 

 camptonectes sculpture, both on the ribs and intervals. The ribs do not 

 increase in number vv^ith age, but become broader and more flattened. 

 Auricles large and broad, thin, corrugated. Byssal notch large and deep. 

 Pectinidial teeth prominent. Hinge-plate thin and but little differenti- 

 ated. Cardinal ridge thin and small, close to the ligament, crossed by 

 fine incisions. The resilial pit is small, but projects beyond the thin hinge- 

 plate in the left valve. 



This is a peculiar group, remarkable for its thin but strongly corru- 

 gated oblique shells, w^ith fine camptonectes sculpture. 



C. JMonotimeris (Con.), from the California coast, is the only 

 species studied. 



Additional diagnosis: Shell moderately to very thin, but translucent 

 only in type species. Ribs often rather angulate, especially in mature 

 specimens. Interior surfaces of interspaces often flat and angulately 

 ridged, except in type species. Sculpture delicate and variable. 



Remarks: Although Verrill based Leptopecten on a single species, 

 he supplied a unit to which we can logically refer a number of 

 closely related species. The subgenus is characterized by thin, broadly 

 rounded shells, slightly to extremely oblique, with long hinge line and 

 large auricles. The following species also belong to Leptopecten : the 

 eastern Pacific latiauratus Conrad, pabneri Dall, velero Hertlein, velero 

 biolleyi Hertlein, euterpes Berry and tumbezensis d'Orbigny, and the 

 western Atlantic bavayi Dautzenberg and linki Dall. This subgenus is 

 apparently restricted to North and South America. 



Key to the eastern Pacific species of Leptopecten 



1. 17 to 19 ribs; known maximum altitude 7.1 mm; ribs low and 

 broad, with alternate ribs stronger; disks unsculptured 



euterpes (Berry) 



(Acapulco, west Mexico) 

 1. 16 ribs or less 



2. Shell very thin and translucent (or nearly so) ; disks un- 

 sculptured (interspaces very infrequently having distant con- 

 centric lamellae) 



latiaurata monotimeris (Conrad) 



(Monterey Bay, California, to Cape San Lucas, Lower Cali- 

 fornia) 



