197 



Distinguishing Characters. — Sinn 11 to medium size : slightly 

 convex, often almost flat; extremities of the hinge line 

 usually prolonged; shells very thin; surface bearing fine, 

 nearly equal striae, increased by intercalation and bifurca- 

 tion, and crenulated bv fine concentric striae; muscular 



i\,& 



Fig. 100. Stropheodonta perplana'. Three pedicle valves ; interiors of pedicle and bra- 

 chial valves, showing muscular impressions, crenulated hinge areas, and cardinal process ; 

 longitudinal section ; enlargement of surface stria? (from Hall). 



impressions of pedicle valve consist of large flabellate divar- 

 icators, spreading in front and extending more than half 

 the length of the shell, and small adductors between ; 

 in the brachial valve strong, rounded ridges, curve 

 forward and outward from the bifid cardinal process, 

 limiting the muscular impressions, while a similar ridge 

 divides them ; interior strongly pustulose. 



