104 
GEOLOGY OF THE FOURTH DISTRICT. 
SHELLS. 
Strophomena (Leptcena), and Delthyris ( Spirifer ), are the most abundant forms usually 
seen. One or two species of Atrypa and Orthis are scarcely less numerous. 
The following are the more common forms of Strophomena: 
35. 
1. Strophomena subplana. 3. Strophomena striata. 
2. S. depressa (lower valve). 4. S. transversalis. 
1. Strophomena subplana (Conrad, Jour. Acad. Nat. Sci. Vol. 8, p. 258). This shell 
is distinguished by the sharp radii which alternate in size, and by the nearly flat valves. It 
often occurs in beautifully preserved specimens upon the surface of thin calcareous layers 
in the shale. When imbedded in shale, the radii are not so prominent. It is the largest 
species of this form in the group, but is often smaller than the figure. 
Locality —Lockport, Niagara, Rochester and Wolcott. 
2. Strophomena depressa. (Productus depressus, M. C. t. 459. Leptcena depressa, Dalm. 
Swedish Transactions, p. 106, t. 1, f. 2; His. Pet. Suec., p. 69, t. xx, f. 3. Murchison, 
Silurian Researches, p. 623, pi. 12, f. 2.) — This shell is abruptly deflected in front; the slight¬ 
ly convex or flat surface marked by six to eight folds or undulations, which are crossed by nu¬ 
merous striae. The sides of the shell at the extremities of the hinge are expanded into wings. 
This portion of the fossil is usually obscured by the matrix, which when removed displays 
the form as figured. 
The deflected portion of the shell is not as long, and the undulations not as numerous as in 
the figure of Mr. Murchison. Specimens from the same rock however, apparently of different 
ages differ in these particulars. The fossil commences its existence in the Clinton Group, 
becomes very abundant in the shale of the Niagara Group, and continues through several of 
the higher formations. It occurs in every locality of the Niagara shale, though usually smaller 
than the one figured, and often apparently more circular. 
3. Strophomena striata, shell semi-elliptical, lower valve slightly convex, upper one flat, 
finely and equally striated, the striae dividing; a few punctulations appear near the margin of 
the shell, which were probably furnished with minute spines. The valves are usually sepa¬ 
rated and spread over the shaly laminae. Its delicate striated surface distinguishes it from 
others of the group. Locality —Lockport; Rochester. 
