SPIRIFER.E OF THE UPPER HELDERBERG GROUP. 205 



Dorsal valve moderately convex or nearly flat, with beak little elevated 



and not incurved : area narrow and straight ; mesial fold smaller than 



the adjacent plications. 



The surface is marked by five or six obtusely subangular plications on 

 the ventral valve, there being sometimes an unequal number on the two 

 sides. The plications of the dorsal valve are broader and rounded, not 

 reaching to the apex ; the mesial fold being smaller than the adjacent 

 plications, depressed below their plane, and the sinus on each side wider 

 than the fold itself. The plications on each side of the sinus of the ven- 

 tral valve are much stronger than the others, and reach to the beak, 

 while the others do not. The finer surface markings are unknown, all 

 the specimens seen having been exfoliated. 



This species is readily distinguished from all the others in our strata, by the 

 extreme disparity of the valves, and the broad and short plications, while at the 

 same time it has not the characters of Oirtlna. 



The figures of this species are enlarged to two diameters. 



Geological formation and locality. This species is only known to me in the 

 Comiferous limestone in the neighborhood of Williamsville and Clarence-hollow, 

 Erie county, New-York. 



Spiiifera yaricosa. 



PLATE XXX. 



Spiri/er varicomt .• Hxll, Tenth Report on the State Cabinet, p. 130. 1857. 

 " " Description of New Pal. Fossils, p. 90. 1857. 



Shell somewhat semicircular or semielleptical ; length equalling or less 

 than half the width : hinge-line equal to the greatest width of the 

 shell, and terminating in salient angles or mucronate extensions. Sur- 

 face plicated. 



Ventral valve much the more convex ; greatest elevation at the umbo, 

 and regularly curving to the front and lateral margins : mesial sinus 

 strongly defined, rather flat in the bottom ; beak slightly arcuate. Area 

 high, nearly flat below and slightly concave towards the apex. 



Dorsal valve moderately convex, with a prominent abruptly elevated 

 mesial fold, which is flattened on the summit and sometimes slightly 



