290 AXEL A. OLSSON 



Subfamily VENERINAE Menke, 1830 



Shell cordate, subtrigonal to well rounded, generally solid, often convex, 

 and with prominent full umbones and prosogyrous beaks. Surface rarely 

 smooth and glossy, more often marked with coarse, growth incrementals or 

 large, concentric lamellae, radial riblets or both. Hinge plate heavy, usually 

 with three cardinal teeth in each valve, sometimes supplemented by a 

 small, rudimentary anterior lateral tooth. Lunule and escutcheon well de- 

 fined, although wanting in some cases. Ventral margins crenulated. 



Key to genera and subgenera of the Venerinae 



I. Hinge with three cardinal teeth in each valve but without an anterior 

 lateral tooth. Pedal muscle scar below the anterior cardinal tooth is 

 not deeply sunken. 



A. An escutcheon is present in both valves. 



Aa. Escutcheon is large, flattened or bevelled, equal in each valve. Pallial 

 sinus small and short, hardly extending beyond the line of the posterior 

 adductor scar. Anterior cardinal tooth in the right valve is small. Shell 

 usually of trigonal shape and of solid texture. 



Genus Chione 



1. Shell trigonal in shape, the sculpture strongly cancellated by inter- 

 secting radial riblets and concentrics of nearly equal strength. 



Subgenus Chione, s.s. 



2. External sculpture formed by large concentric riblets, solid, and 

 rounded over the middle of the disk, lamellose on the two ends. 



Subgenus Lirophora 



3. External sculpture formed by large, rounded, concentric undulations, 

 pronounced on the umbones but become more or less obsolete ventrally. 

 Radials sometimes present also. Color pattern variable. 



Subgenus Iliochione 

 Ab. Escutcheon unequal, smaller in the right valve. The pallial sinus is 

 large and deep. Anterior cardinal tooth in the right valve large. Shell 

 cordate, thinner, the posterior end often produced and pointed. 



4. Sculpture subreticulated, often with the concentric lamellae elevated 

 and elaborately frilled or scalloped. 



Genus Chionopsis 



B. An escutcheon is absent or restricted to the left valve. 



Ba. The escutcheon, restricted to the left valve, is narrozv, flattened. Pos- 

 terior-dorsal margin of the right valve is often folded so that it over- 

 laps the margin of the other at the distal end. Hinge like that of Chione, 

 the right anterior cardinal tooth is small. 



5. External sculpture formed by coarse, rounded, close-set, radial riblets 

 crossed by widely spaced, concentric lamellations on the sides. Shells 

 rounded and convex, the surface color pattern generally mottled. 



Genus Notochione 



6. External sculpture formed by much smaller radial riblets, noded, or 

 cancellated by the concentrics. 



Genus Protothaca 

 6a. Shell relatively large, white or drab in color, the external sculpture 

 like that of a Corbis or Fimbria, the concentrics heavier on the an- 

 terior side. Cold-water. 



Subgenus Prothothaca, s.s. 



7. Shell smaller, the surface more highly colored, often with a varied and 

 elaborate pattern. Warm waters. 



Subgenus Tropithaca 



