LOWER HELDERBERG ROCKS. 201 



Spirifer perlamellosus (n.s). 



Plate XXVI. Fm. 1 & 2. 



Spirifer rugosa : Hall in Catalogue. 



Shell trigonal or semicircular, more or less extended on the hinge line, 

 the extremities varying from obtuse or rounded to extremely mucro- 

 nate. Ventral valve arcuate, the beak much extended beyond the 

 opposite valve, and incurved at the apex : sinus deep, gradually ex- 

 panding, and produced in front into a linguiform extension. Dorsal 

 valve convex towards the middle, the mesial elevation very prominent, 

 and the beak closely incurved against the area, or partially closing the 

 foramen of the ventral valve. Area moderately. wide, frequently much 

 expanded, and becoming linear towards the extremities when the 

 shell is much extended. 



Surface marked by four to six strong and abruptly elevated plications 

 on each side of the mesial sinus and elevation, concentrically marked 

 by strong imbricating lamell89, which are abruptly arched in passing 

 over the plications, giving an extreme roughness to the surface. In 

 well-preserved specimens, finer longitudinal striae mark the surface of 

 these lamellae. In ordinary specimens, the concentric lamellae are more 

 closely arranged and more distinctly imbricate towards the margin ; 

 while near the beaks they are ijiore distant, and are scarcely imbricate. 

 This species, in its young state, closely resembles the S. sulcatus of the Niagara 



group; but this shell is more rugose, and the lamellae less arched on the plications, 



which are also less numerous in specimens of the same size. The area, in both 



species, is subject to much variation. 



The form of this species varies mainly in the greater or less extension of the 



cardinal extremities. 



Fig. \ a — s. Illustrations of gradations of size and diflFerences of form presented by this species. 



Fig. t. Enlargement of surface, showing the concentric lamellae [which are not strong enough 



in the figure] and the fine longitudinal striae. 

 Fig. 2 a, h. Interiors of ventral valves. Fig. 2 c, d, e. Interiors of dorsal valves. 



Fig. 2 f, g. Cast of the interior, and enlargement of the papillose surface. 



Geological position and locality . In the shaly limestone of the Lower Helderberg 

 group, and rarely in the Pentamerus limestone : Helderberg mountains; Schoharie, 

 Carlisle, Catskitl, Hudson, Cherryvalley, etc. 



f PALiEONTOLOaT III.] 26 



