SPIRIFEK.E OF THE UPPER HELDERBERG GROUP. 193 



distinct, with a subnodose character, on the exfoliation of the shell. The 

 mesial sinus, is usually very wide at its base, sometimes equal to half 

 the length of the shell. 



The proportions of the shell are extremely variable, the length being 

 sometimes greater than the width, while usually the width is somewhat 

 greater than the length ; and in some individuals the length and breadth 

 are as two to three. 



The casts of the ventral valve show a small rostral cavity with short 

 strong dental plates ; the muscular area being small, quadrangular, and 

 divided through the middle by a distinct septum. 



The interior of the shell of the ventral valve shows short strong hinge- 

 teeth and very short incurving plates below, while the bottom of the 

 cavity is divided by a distinct elevated septum. 



The dorsal cast shows marks of dental sockets, with strong muscular 

 markings at the apex of the fissure. 



This species is nearly allied with S.perlamellosa oi the Lower Helderberg 

 gi'oup, and is not always readily distinguished from that one. It is, however 

 usually proportionally shorter on the hinge-line, and more quadrate in form, 

 while it is not known to have as many plications as the older species. 



The dimensions vaiy from half an inch in length and breadth, to an inch and 

 a half in length and breadth ; while other specimens are an inch and a half in 

 width by an inch in length. 



This species is not common in the rocks of New York, though having a pretty 

 wide distribution. 



Geological formations and localities. This shell occurs at the Helderberg 

 mountains and Schoharie, in the Schoharie grit ; and in the same region, in the 

 Cornifcrous limestone. It is known in the latter rock at Stafford, Caledonia, Wil- 

 liamsville in Western New- York ; Columbus, Ohio ; Falls of the Ohio river ; and 

 in several localities in Canada West. 



[ PaljEontolooy IV.] 26 



