A. E. Verr ill— Study of the family Pectinidce. 69 



Although this group agi'ees with Anmsium in having internal ribs 

 without corresponding external grooves, it seems to be allied rather 

 to Chlamys. It may be regarded as a division of the latter in which 

 the external radial ribs have degenerated. 



It includes C. hyalinus (Poll), from the Mediterranean, and several 

 other similar small species. 



Leptopecten, new subgenus of Chlamys. Type, C. Monotimeris (Conrad). 



Shell thin, translucent, oblique, broadly rounded, with strong, 

 rounded radial ridges or folds, like corrugations, which 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 with fine divergent camp- 

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

 increase in number with 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 

 differentiated. Cardinal ridge thin and small, close to the liga- 

 ment, 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 coi^- 

 rugated oblique shells, with fine camptonectes sculpture. 



C. 31 onotitneris (Con.), from the California coast, is the only 

 species studied. 



Placopecten, new subgenus of Chlamys. Type, P. Clintonius (Say). 



Plate XVII. figs. 1-7. Plate XX. figs. 7, 8, 8a, Plate XXL figs. 1, la, 2, 2a, 



Shell large, compressed, broadly rounded, rather thin, with simple 

 sharp edges, meeting evenly ventrally, but gaping considerably at 

 both ends, especially when adult (pi, xvii, fig, 5), Valves only 

 slightly unequal in form, the right one being a little flatter, but they 

 differ in color and somewhat in sculpture, the right one being 

 smoother and paler. Both have fine radial lines or riblets, and they 

 have vermiculated divergent riblets when young. Auricles small, 

 symmetrical, nearly equal. Byssal notch small, simple. Pectinidial 

 teeth generally obsolete, except when young. No internal ribs. 

 Inner surface often with more or less pearly luster and a crystalline 

 structure. Hinge-plate with two feeble, slightly divergent ribs on 

 each end, crossed by fine transverse incisions. The foot (pi. xx, fig. 

 8) is well developed, oblique, slightly narrowed distally and enlarged 

 at the end, where it is divided into two lobes by a rather deep, 



