Bracldopoda of the Cincinnati Group. 59 



Dorsal valve Avitli a shallow, midefincd mesial sinus of moderate 

 l^readth at the front, but narrowing posteriorly and usually fadino; out 

 before reaching the umbo ; surface on each side of the sinus gently 

 convex centrally, and sloping gradually to the lateral margins ; beak 

 but slightly prominent and incurved. 



Ventral valve with a more or less elevated mesial ridge, generally 

 most prominent near the middle, and somewhat depressed anteriorly, 

 while the slopes on each side are distinctly compressed ; beak small, 

 abruptly pointed, perforated, projecting beyond that of the other valve, 

 and rather distinctly arched ; margin on each side of beak carinated, 

 so as to give the appearance of a kind of false cardinal area. 



Surface of each valve ornamented by about eighteen simple, rounded 

 plications, of which one at the umbo and five at the front of the dorsal 

 valve occupy the mesial sinus ; the one extending from the umbo 

 (or central plication) is the largest, and corresponds with a deep 

 furrow in the middle of the mesial ridge of the ventral valve ; about 

 four plications occupy the mesial elevation of the ventral valve, two 

 on each side of the deep central furrow ; marks of growth obscure and 

 surface obscurely punctate. 



Found throughout the Cincinnati Group, at all elevations, in great 

 abundance, and in a good state of preservation. 



Zygospira Cine innatietms — (James) . 



This variety or species differs from the last in its larger size, greater 

 proportional breadth, more prominent mesial elevation, deeper furrow 

 in the middle, deeper mesial sinus, and more spreading and coarser 

 lateral plications. Its lateral margins are generally much compressed. 



Prof. Meek thinks it may be only a robust variety of Z. modeda. 



Found on the hills back of Cincinnati, associated with the modesta, at 

 elevations from 250 to 400 feet aljove low water-mark. 



Zygospira headi — (Billings, 1862). 



Shell attaining a medium or rather large size, longitudinally oval, 

 the front being regularly rounded, and the lateral margins more 

 broadly rounded, or sometimes slightly straightened along the middle ; 

 valves both rather evenly convex. Dorsal valve a little less convex 

 than the other, its greatest prominence being near the middle, but 

 without any traces of a mesial ridge ; beak very short and incurved. 



