90 A. E. J^errill — Study of the family Pectinidm. 



e. Shell swollen, valves stroDgly convex, nearly smooth, not hyaline; auricles 

 unequal, oblique. Pectinella. 



ee. Shell compressed, valves little convex, often hyaline. 

 /. Both valves undulated, and with fine radial sculpture or smooth. Texture 



hyaline. Hyolopecten. 



ff. Valves not undulated, sculpture none, or of fine, radial riblets, or cancellated on 

 one or both valves, or vermiculated. 

 g. Shell nearly smooth, or with small radial riblets; divergent vermiculated sculp- 

 ture may be present, at least when young. Fseudamusmm (sens, restr.) 

 gg. Shell more or less hyaline or translucent ; vermiculated divergent sculpture con- 

 spicuous when adult, either with or without radial riblets or rows of scales ; auri- 

 cles uuequal. Gamptonectes. 

 • Pnlliolum. 

 del. Valves with the edges smooth and unlike, that of the lower one flattened or 

 bevelled and shutting against the upper one, sculpture on the lower valve con- 

 sists of concentric raised hues or riblets ; on the upper valve of radial riblets, 

 or striae, lines of scales, or cancellations. Cyclopecte7i. 

 GG. — Anterior auricle without a byssal notch. Shell thin, rounded, symmetrical. 

 h. Auricles of one valve, prolonged dorsally in distinct angles or points. Entolium. 

 hh. Auricles not prolonged dorsally ; sculpture radial on one valve. Syncyclonema. 

 hhh. Auricles nearly equal, angular, not distinctly prolonged dorsally. Shell very 



thin, with fine concentric sculpture. Protamusinm. 



BB. — Shell with special internal, radial ribs, independent of external sculpture. 

 Smooth or with delicate sculpture externally ; not ribbed. Edges of valves not 

 scalloped. Body of shell usually broadly rounded, not oblique. 

 i. Shell nearly closed at both ends ; texture hyaline ; auricles both well developed ; 



byssal notch and pectinidial teeth present. 

 j. Valves nearly equal; lower valve not concentrically sculptured; both valves 

 with fine radial lines ; internal ribs numerous, simple, not differing from the shell 

 in texture. Lissopecten. 



jj. Valves unequal ; lower valve strongly concentrically sculptured ; upper valve 

 with radial lines, or rows of scales, or cancellated; internal ribs few, well differ- 

 entiated from the shell. • Frojiearnvsmrn. 

 it. Shell large, thin, round and fiat, gaping at both ends, usually smooth and polished, 

 but not hyaline ; auricles feebly developed ; pectinidial teeth and byssal notch 

 obsolete. Internal ribs strong, usually opaque white. 

 k. Two pairs of gills. Amushim. 

 kk. One pair of gills. Paramusium. 

 AA. — Hinge-plate with a series of definite pits, resembling resilial pits, on each side 

 of the central pit. 



I. Auricles small, not well defined. Shell smooth or with fine radial sculpture. 



Pernopiecten. 



II. Auricles well developed, angular. Sculpture concentric. Euchondria. 

 II. — Hinge-plate without a central resilial pit. 



m. Shell nearly symmetrical. A series of small pits, resembling resilial pits, along 



the hinge-margin. Crenipecten. 



mm. Shell oblique, inequilateral, auricles unequal, ligament confined to a groove. 



Avkulopecten. 



