72 
PALEONTOLOGY OP NEW-YORK. 
These variations are illustrated on Plate ix, where figures 1 & 2 are typical forms of the 
species, in some of which the area is inclined forwards. 
Pigs. 3, 4 & 5 are views of dorsal valves which are proportionally longer, but have a regular 
convexity. 
Figs. 6 & 7 are of a larger dorsal valve of similar form, but more irregular exterior. 
Pigs. 8 & 9 are the dorsal and ventral valves of a slightly distorted specimen. 
Fig. 10 is a ventral valve which is slightly unsymmetrical. 
Pig. 11 is a symmetrical ventral valve, which has the form of area and muscular impression 
of S. pandora. 
Pig. 12 is an enlarged figure from a cast in arenaceous shale. The striae appear coarser than 
in the softer shale. 
In a slab with numerous specimens, the apices of some are a little distorted, and others are 
quite symmetrical. 
Geological formation and locality. This variety occurs in the Hamilton group, 
at York, Moscow, Pavilion, Darien, Canandaigua lake, and Bristol, Ontario county; 
and near Tully, Onondaga county. 
Var. C. Streptorhynchus perversa. 
PLATE IX. 
Orthis perversa : Hall, Tenth Eeport on the State Cabinet, 1857, p. 97. 
Orthisina alternata : Hall, Thirteenth Eeport on the State Cabinet, 1860. 
Shell subelliptical, wider than long; the cardinal extremities rounded, 
and the hinge-line less than the width of the shell. Length and width 
about as two to three; the sides curving shortly, and the front broadly 
rounded. The ventral valve is very convex at the umbo and the beak 
distorted, somewhat depressed below the umbo, and nearly flat or un¬ 
equally depressed-convex on the middle and lower part : area nearly 
vertical, with the apex inclined or arcuate ; foramen closed by a pro¬ 
minent convex pseudo - deltidium. Dorsal valve rounded, convex or 
sometimes gibbous; the greatest convexity above the middle, and 
sloping abruptly to the hinge-line, sometimes flattened or a little de¬ 
pressed in the middle towards the front. 
Sueface marked by distant elevated striae which increase mainly by 
interstitial additions, and the intermediate spaces are crossed by ele¬ 
vated concentric undulating strise. 
In my description of this species, I remarked that it “is of the type of Orthis 
“ umbraculum , and presents the irregularities of form common to that shell,” 
