Gems DIELASMA. Ki.nc;. 



[Ety. : Di, two ; da.s/iui, lamella.] 



(IS.-)!) : Pioc. I)ul)lin Univ. Bot. Zool. Assoc, Vol. I., p. 260. 1898: Pal 

 N. v., Vol. VIII., p. 2S»8.) 



Terebi-atnloid sliells, witli frequently a niediaii sinus in 

 hotli valves. A large foramen and deltidial ])lates ai-e })res- 

 ent. thoutih these latter may l)e oltseured by the incurvature 

 of the beak. A large hinge jjlate, raised but little above the 

 bottom of the valve, and sometimes actually adhering to it, 

 and a relatively short brachidium, with a rather short 

 recurvature of the ascending lamelhe, are characteristic 

 internal featui-es. 



The sul>-genus Cran.ena ( Hall), was erected to receive such 

 forms as Terebratula roinini>eri (Hall), in which the brachi- 

 •bum is that of a Dielasma, but the hinge ])late reseml)les 

 that of Crvptonella. 



I)lEL.\SMA (("rAX.J'LXA) I;( ).M I.\( iERI. 



(Pal. X. Y., Vol. IV., p. 889, PI. LX.) 



D is t in g 11 is li in g Chnnicters. — 

 Small size; ovate outline; trun- 

 cated or slightly sinuate front; 

 rounded cardinal slo})es; concentric 



. in- V\G. 149. Dielasma romiiigeri. 



Strue, which are often crowded mtO r)orsal. ventral, and profile views 



(from Hall). 



wrinkles near the fi-ont. 



Found in the Pleurodictynm beds and the Calcareous bed 

 above, on the Lake Shore, and in Averv's Fiavine. 



Hall. (Fig. 141).) 



Genus TROPIDOLEPTrS. Hall. 



[Ety.: Tropis, keel; Icptos, slender.] 



(1857: 10th Ann. Rep't N. Y. State Cab. Nat. Hist., p. 151. 1898: Pal. 

 N. Y., Vol. VIII., Pt. II., p. 302.) 



Shell coiicavo or plano-convex, with the hinge line much 

 extended and straight, the whole aspect of the shell l)eing 

 ''Strophomenoid" (ccjm])are Stropheodonta). In young 

 shells the liinge line equals or exceeds in length the greatest 

 width of the shell, but in mature shells it is shortei'. Pe(hcle 



