26 Cincinnati Society of Natural History. 



margin broacll}', but regularly rounded; posterior end rather nar- 

 rowly rounded. 



Surface unknown. The interior, as shown in casts, presents a deep 

 triangular groove just anterior to the beaks, and passing, for a dis- 

 tance a little more than one third of the heighth of the shell, toward 

 the anterior basal margin; the anterior muscular scar is distinctl}^ de- 

 fined, ovate, and situated immediately in advance of the groove; the 

 posterior scar is rather large, sub-circular, and placed on the umbonal 

 ridge (in the cast of the interior) about in the middle of the distance 

 from the beaks to the posterior basal margin; pallial line obscure, 

 simple, and running nearly parallel with, and considerablj' within, the 

 basal border. 



This is the largest species of the genus. 



It is most nearly related to C. elongatas, Hall, from Nova Scotia. 

 That shell however is sinuate, has the cardinal margin rounded and 

 not straight, the beaks closer to the anterior margin, and has the clav- 

 icle bent and proportionally longer than in our species. 



Formation and locality: Hudson River Group. Not rare on the hills 

 back of Cincinnati, O. 



CRISID^. 



ROPALONARIA, UOV. gCU. 

 [Ety.— From ropalon, a club.l 



Polyzoary creeping, adnate, branched, and forming a close and del- 

 icate network. Branches linear; cells uniserial, elliptical, joined to- 

 gether at their contracted ends. This genus is related to Hippothoa, 

 but in the form and arrangement of the cells they differ widel}'. 



RopALONARiA VENosA, n. sp. (Plate VII., figs. 24, 24a.} 



{Eiy .— Venosns, full of veins.] 



Polyzoary creeping, adnate, branched, and forming a very delicate 

 network. Branches linear, with a straight central stripe or series of 

 cells, which has two branches springing, usually from every junction 

 of the cells, though sometimes at that of the second with the third; 

 these branches are again divided in a similar manner, and anastomose: 

 this peculiar mode of growth gives the polyzoary very much the ap- 

 pearance of the venation in a leaf. Cells uniserial, long, acutely ellip- 

 tical, and joined together at their contracted ends, length of cells 

 somewhat variable, but generally about four occupj- the space of two 



