LOWER HELDERBERG ROCKS. 205 



This species belongs to the same group as the preceding ; differing from the Nia- 

 gara S. radiatus in its higher area and rounded cardinal extremities, as well as in 

 the depressed rounded folds upon the surface. If, however, we regard the S. cyrtmia 

 of Dalman as identical with the species from the Niagara and Clinton groups, the 

 presence or absence of plications would appear not to be of specific importance. 

 In this one, however, this character is combined with so many others, as to render 

 the distinction very well marked. 



Fig. 3 a, h, c. Ventral, dorsal, and profile views. 

 Fig. 3 d. Enlargement of the surface striae. 



Geological position and locality. In shaly limestone, associated with many species 

 common to the Lower Helderberg group : Decatur county, Tennessee. 



Spirifer octocostatus ( n. s.). 



Plate XXVIII. Fio. 4 a - e. 



Shell subglobose : valves nearly equally convex. Ventral valve most 

 elevated near the beak : sinus angular, extending to the apex : beak 

 slightly incurved. Dorsal valve most convex in the middle : mesial 

 elevation not prominent : beak rising little above the hinge line, 

 slightly incurved; hinge line less than the width of the shell, rounded 

 at the extremities. Area triangular, faintly defined, somewhat arcuate. 

 Foramen narrow ; a strong median septum dividing the muscular area, 

 and extending to the apex of the foramen. 



SoEFACE having about four rounded moderately prominent folds on each 

 side of the mesial sinus and elevation, which become obsolete towards 

 Ihe beaks ; concentrically marked by fine, regular, closely arranged, 

 imbricating lamellose striae. 



This species resembles S. crispus of the Niagara group, but is more globose in 

 form, has a much shorter area, more rounded extremities, and less strongly marked 

 plications. It appears to be intermediate between that species and S. bicostatus of 

 the same group, but is readily distinguished from either of them. 



Pig. 4 a, b, c. Ventral, dorsal, and profile views. 



Fig. 4 d. Cardinal view, showing the foramen. Fig. 4 e. Interior of the ventral valve. 



Geological position and locality. Limestones of the Lower Helderberg group : 

 Cumberland (Md.). 



