182 
GEOLOGY OF THE FOURTH DISTRICT. 
This is a very abundant little shell, often forming thin courses, with scarcely the interven¬ 
tion of sufficient shale to make them cohere. It much resembles the Spirifera unguiculus 
(Phil. Paleozoic Fossils, p. 69, fig. 119), and Orthis umbonata (Conrad, Jour. Acad. Nat. 
Sci. Yol. viii, p. 264, pi. 14, fig, 4). The beak is much less prominent, the hinge line less 
extended, and the shell smaller than the last named shell. 
9. Orbicula minuta , n. s. — Orbicular; surface marked by concentric lines. 
This fossil is exceeding abundant, often nearly covering the shaly laminae for several inches 
in thickness. They appear like small black specks or points. The larger figure is its maxi¬ 
mum size. It is remarkably persistent in the district, being known at the most extreme loca¬ 
lities. 
Localities — Avon, Le Roy, Bloomfield and numerous other places. 
10. Tentaculites jissurella, n. s. — Minute, almost microscopic; annulated above, and 
smooth near the base. When sufficiently magnified, a slit is perceived on one side, extending 
one-third or more of the whole length. 
This fossil occurs in myriads, and, although so exceedingly minute, it forms layers several 
inches thick, extending many yards, and apparently many miles, it being equally abundant 
at distant localities. The layers which it forms are exceedingly fragile, and crumble on the 
least exposure to moisture. 
Localities — Avon, Le Roy, Waterloo, Alden. 
11. Atrypa limitaris. (Orthis limitaris , Geol. Report of Third District, p. 146, fig. 3.) 
Compressed, somewhat circular; surface covered with radiating ribs of nearly equal size. 
This shell is exceedingly abundant in the upper part of the shale ; the stratum, for conside¬ 
rable thickness, is completely charged with them, and they are often associated in great num¬ 
bers with Orbicula minuta. The specimen figured is a fragment of this kind, which presents 
the fossil as it usually appears in the rock. It will be readily recognized. 
Localities — Waterloo, Vienna, Avon, Le Roy. 
In addition to these fossils, there are two or three univalves which occur everywhere; one 
of these is a Goniatite, but from being replaced by iron pyrites which is in a decomposing 
state, it is impossible to define it. The large Goniatites figured in the Report of the Third 
District, page 146, as occurring in this rock, have not been seen in the Fourth District. 
