260 TRANSACTIONS OF THE [MAB. 15, 



and incurved so as to bring the circular foramen nearly on a line 

 with the margins of the valves. Area small, triangular, bounded 

 by a distinct angular margin. 



Brachial valve smaller than the pedicle valve, beak small, 

 scarcely projecting beyond the cardinal margin. 



Surface ornamented with about fifty simple, rounded, longi- 

 tudinal plications. 



None of the specimens of this shell in the collection are perfect 

 enough to figure. So far as preserved they agree exactly wiih 

 Hall's and Whitfield's figures of the species. 



Dielasma turgida Hall, var. elonqata var. nov. 

 PI. XIX., fig. 6. 



Shell small, elongate, sub-fusiform, as thick as broad. The 

 valves rather strongly convex, their junction obtuse, emarginate 

 in front. Surface smooth. 



Pedicle valve furnished with a deep median sinus throughout 

 nearly its entire length, growing shallower as it approaches the 

 beak. Beak narrow, slightly incurved. 



Brachial valve less convex than the pedicle valve, pro'/ided 

 with a median sinus, but shallower than that in the pedicle valve. 



This shell is distinguished from the typical D. turgida H.,* 

 from the Spergen Hill beds in Indiana, by being more elon- 

 gate and less ventricose. The European shell D. sacculus is a 

 closely allied form, but is more nearly like the typical D. tur- 

 gida than the Arkansas variety. 



MOLLUSCA. 



Pelecypoda. 



Pinna arkansana sp. nov. 



PI. XX., figs. 1-2. 



Shell acutely triangular in outline. Towards the beak, regular- 

 ly convex from the hinge-line to the ventral margin, depressed 

 posteriorly. 



Hinge-line straight, equal to the greatest length of the shell. 

 Just below the hinge-line and parallel to it is a well-defined 

 but shallow furrow, separating a low rounded marginal ridge 

 from the body of the shell. Ventral mai-gin straight or but 

 slightly arcuate towards the beak, meeting the hinge-line ante- 



* Terebratula turgida, Hall. Trans. Alb. Inst., Vol. 4, p. 6. 

 Terebratula turgida, Whitfield, Bull. Am. Mus. Nat. Hist., Vol. 1, p. rA, PI. 0, 

 figs. 53-58. 



