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GEOLOGY OF THE FOURTH DISTRICT. 
1. Atrypa dumosa, n. s. — Semicircular, or somewhat transversely elliptical; beak of the 
flatter valve scarcely prominent; surface marked by about 32 rounded radiating ribs; the 
ribs are crossed by elevated, concentric, undulating, thread-like lines, and at more distant 
intervals, elevated lamellae ; the lamellae upon each rib are folded into a short round spine. 
Fig. 1 and 1 a, are faithful representations of two specimens, the latter imperfect. 
1 b. Cast of the interior of the flat valve, punctured throughout, except the subrostral im¬ 
pression. 
This fossil is abundant, though the spines are usually removed, and the surface presents a 
squamose appearance, which had induced me to refer it to A. squamosa of Sowerby. 
Localities —Chemung ; Cayuta creek ; Elmira. 
2. Atrypa hystrix, n. s. — Transversely subelliptical, often approaching to circular; beak 
scarcely prominent; surface marked by about eight broad ribs, which are crossed by five or 
six elevated lamellae; at the junction of these with each rib, proceeds a long slender spine. 
This fossil is totally distinct from any form of Atrypa figured, and is one of the most 
striking fossils in the rocks to the south of Bath, Steuben county. 
3. Atrypa tribulis, n. s. — Hemispherical; upper valve very convex, with the lateral edges 
pressed downwards ; lower valve nearly flat, elevated in front, and reflexed at the sides ; 
beak rather prominent; surface marked by numerous divaricating radii, which are crossed by 
few concentric lines of growth. 
3, lower valve. 3 a, upper valve. 3 b, cast of the interior of the flat valve. 
There is considerable resemblance between the cast of this fossil and that of A. dumosa 
(1 b), but on closer comparison, they will be found to differ essentially. In this species, the 
cast of the subrostral depression is longer and less strongly defined; the subrostral plates 
are likewise compound, while they are simple in A. dumosa. This will be seen by the small, 
projecting, tooth-like process, extending into the space on each side the beak, and which 
shows a division of the plates, which were proportionally larger than in the other species. 
Unlike as these fossils appear, it is often difficult to distinguish them under different 
aspects, when imbedded in a different matrix, and in various stages of perfection. The casts 
of these, as well as other species of the genus, will be found reliable. 
There is a very close resemblance between this fossil and the forms usually refered to A. 
prisca, A. affinis, &c., and I was disposed to refer it to that species ; but further examination 
has induced me to consider it distinct, until an opportunity offers of examining the internal 
structure of those analogous or identical species in the lower rocks. 
Localities —Ithaca; Chemung; Elmira. 
4. Atrypa tenuilineata, n. s. — Nearly circular; beak small; surface marked by numerous 
very fine radii. 
It is possible that this fossil is an Orthis. 
Locality —Cattaraugus county. 
