SPIRIFERJE 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 Cyrtina. 
The figures of this species are enlarged to two diameters. 
Geological formation and locality. This species is only known to me in the 
Corniferous limestone in the neighborhood of Williamsville and Clarence-hollow, 
Erie county, New-York. 
Spirifera varicosa. 
PLATE XXX. 
Spirifer varicosus : Hail, 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 
