STREPTORHYNCHUS. 
*3 
The species described as Orthisina alternata is not distinct from this; and al¬ 
though without larger collections of specimens, it is impossible to draw lines of 
specific distinction, yet the prevailing forms may be separated into two groups; 
leaving a few which are not so readily referred to either of them, or which may 
be intermediate forms, uniting the two and likewise including S. pandora. 
I have adopted the name S. pandora as applicable to the symmetrical forms of 
the Schoharie grit and Corniferous limestone, and some of those in the Chemung 
group; while the S. arctostriata and S. perversa are more characteristic of the 
Hamilton group. 
Figs. 18 & 14 are examples of the dorsal valves of this variety. 
Fig. 15 is an irregular form of the same. 
Fig. 16 is an intermediate form of the ventral valve, with the striae intermediate in distance 
and the cardinal angles but little rounded, 
Fig. 17 is the dorsal valve, which is quite symmetrical in form, but with distinct striae and 
rounded hinge-extremities. 
Fig. 26 is a very convex dorsal valve with slightly rounded cardinal extremities, and having 
comparatively distinct striae. The umbo is very prominent; below which it is 
flattened in the middle, and slightly depressed towards the front of the shell. 
Geological formation and locality. This variety occurs on Seneca and Cayuga 
lakes, Moscow and Hamilton, in the Hamilton group. Fig. 17 is from the same 
group, in the village of Fabius, Onondaga county. Fig. 26 is from the Corniferous 
limestone at Williamsville, Erie county. In the latter, the striae are more nearly 
equal. 
Var. D. Streptorhynclms pcctinacea. 
PLATE X. 
Strophomena pectinacea and S. bifurcata : Hall, loc. cit. 
Shells usually symmetrical, often unsymmetrical, semioval, with the 
hinge-extremities sometimes a little salient, often truncate and some¬ 
times rounded. 
The dorsal valve is moderately convex, or sometimes gibbous. The 
surface is marked by distant, very prominent striae, with two or three 
finer ones between them. The hinge, and other features of the shell, are 
the same as in other varieties of the species. 
The well-marked specimens are usually small or of medium size. 
[ Paleontology IV.] 10 
