160 [Senate 



and sinus. Surface marked by very fine obscure radiating striae, 

 which are crossed near the borders by small, closely arranged, 

 subimbricating lines of growth. Fine distant granules are di- 

 stributed over the whole surface, and most conspicuously in the 

 sinus and between the plications. 



This shell may be readily distinguished from S. granulifera, with which 

 it is associated, by its less gibbous form, shallower sinus, more depressed 

 mesial fold, and much less distinctly arcuate beaks. Under a magnifier, the 

 granulations of the surface are always seen to be much more scattered than 

 on S. granulifera ; while the plications are narrower and more numerous, 

 and more abruptly elevated above the general surface. 



This species is among the finest of the Hamilton spirifers, and superior 

 in size and beauty to all except the S. granulifera. 



Geological position and locality. In shales of the Hamilton group : 

 Covington, Genesee county. 



SpIRIFER CORTICOSA ( n. S.). 



Pal. N.Y. Vol. iv. 



Shell semicircular, more than half as long as broad; hinge nearly 

 or quite equalling the greatest width of the shell : dorsal valve 

 unknown : ventral valve convex ; sinus deep and well defined, 

 extending from the apex of the beak to the front, where it ter- 

 minates in a short rounded projection ; beak incurved ; area 

 narrow and arcuate ; foramen small ; plications simple and 

 rounded, numbering about ten to twelve on each side of the 

 sinus. Surface ornamented by very fine closely arranged con- 

 centric striae, and stronger more distant imbricating marks of 

 growth. 



This is a rare species, of which I have yet seen no perfect individuals. 

 In its narrow area, it resembles S. mucro7iata ; but the beak is more 

 arched, the sinus deeper, the plications stronger and less numerous, while 

 the concentric lamellae arc equally strong but less sharply arched over the 

 plications. Single valves may be distinguished by the absence of a slight 

 elevation along the centre of the sinus, which marks the well-preserved 

 specimens of S. mucronata. 



Geological position and locality. In the shales of the Hamilton group, 

 near Darien, Grenesee county. 



