A. E. Verrill — Study of the family Pectlniche. 89 



I. — Hinge-plate with a central resilial pit. 



A. — Hinge-plate without lateral series of marginal pits, resembling resilial pits, but 

 often with small transverse incisions. 



A. — Shell with very unequal valves: the right, or lower, valve very convex with a 

 strongly incurved beak ; left valve nearly or quite flat, shutting closely inside of 

 the edge of the right valve. Both valves with strong primary radial ribs and 

 internal lirte; edges scalloped. Animal not adapted for swimming. 



a. Hinge with the cardinal ribs plain or nearly so. Pecten (restr.) 

 aa. Hinge with the cardinal ribs strongly transversely incised or pitted. 



Neithea (sub -gen.) 



A A. — Shell with the valves not very unequal; the upper or left valve the most con- 

 vex, when any difference exists. Sculpture various. Animal adapted for swim- 

 ming, at least when young. ■* 



B. — Shell internally fluted or smooth, without special radial ribs developed independ- 

 ently of external sculpture. 



C. — Hinge-plate with several large distinct, nearly transverse, tooth-like processes. 

 Shell with large external and internal radial ribs. Byssal notch obsolete. 



Pallium. 



CC. — Hinge-plate without transverse tooth-like processes. Sculpture various. 



D. — Hinge-plate with more than two cardinal ridges, either side of the resilial pit, 

 the lower ones divergent. Radial ribs large and more or less nodose. A dis- 

 tinct byssal notch. Lyropecten. 



DD. — Hinge-plate with only one or two cardinal ridges, the upper one parallel with 

 the dorsal margin and bounding the ligamental groove ; the second, when 

 present, more or less oblique. Sculpture various. 



E. — Shell, when adult, permanently attached by the right valve, which is more or less 

 distorted. A deep byssal notch. 



b. Right valve directly attached by a shelly deposit. Hinnites. 

 bb. Right valve attached by a modified byssus. Eemipecten. 



EE. — Shell free through life, or only temporarily attached by a byssus of thread-like 



fibers. Byssal notch usually present. 

 F. — One or both valves with external primary ribs or riblets. 



a. External ribs small, nearly obsolete on right valve. No internal lirae. Edges 

 of valves not scalloped nor strongly crenulated. Placopecten. 



aa. External ribs and internal lirje strong ; edges of valves scalloped. Byssal notch 

 well developed. 



c. Primary radial ribs and internal lirfe simple, not increasing with age by forking; 

 shell scarcely oblique. .^quiptcten 



CC. Primary ribs and lirse of various sizes, increasing in numbers with age by 

 forking, or by the interpolation of new ones; auricles or shell usually oblique. 



Chlamys (restr.) 

 aaa. External and internal ribs simple, formed by corrugations of the shell ; internal 



ribs not thickened. Leptopecten. 



FF. — Both valves destitute of strong external ribs and internal thickened lira; ; edges 



either smooth or slightly crenulated. 

 G. — Anterior auricle well developed, with a distinct byssal notch. 



d. Valves with the edges simple or slightly crenulated, equal, meeting nearly 

 evenly. 



