197 



DistinguMiin^- Characters.—'^mnW to medium size ; slightly 

 convex, often alnK)st flat; extremities of the hinge line 

 usually ])rolonge(l; shells very thin; surface bearing fine, 

 nearly equal striae, increased by intercalation and bifurca- 

 tion, and crenulated by fine concentric strijp ; muscular 



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i''u\ 



5>i^w^r'y»Tr f.'":r.-fr -J^-T~^'-^r -cvtT-*^.i 



'trcsssEoxosaa 



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 striae (from Hall). 



impressions of pedicle valve consist of large flabellate divar- 

 icators, spreading in front and extending moi-e 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 tliem : interior strongly pustulose. 



