180 
GEOLOGY OF THE FOURTH DISTRICT. 
Organic Remains of the Marcellus Shale. 
Nearly all the fossils are of small size and very delicate structure, indicating, as does the 
finely levigated mud in which they are imbedded, a quiet condition of the ocean in which they 
lived. These delicate forms are preserved in the greatest perfection, the most fragile shells 
being often as perfect as when living. 
This character of small size of the fossils does not apply to the rock everywhere; for in 
the Third District it contains some goniatites, larger than in any other rock in the State. 
The more common fossils met with, and those which are typical of the rock, are presented 
in the following illustration. So far as can be ascertained, these forms are all undescribed, 
with the exception of the last, which was named in the Report of the Third District. These 
occur at the two localities mentioned as the best points of observation, where they are very- 
abundant, as well as many others not here figured. 
71 . 
1. Orthoceras subulatum. 4. S. pustulosa. 7. A. equilatera. 10. Tentaculites fissureila. 
2. Strophomena setigera. 5. Avitula muricata. 8. Orthis nucleus. 11. Atrypa limitaris. 
3. S. mucronata. 6. A. lsevis. 9. Orbicula minuta. 
1. Orthoceras subulatum, n. s. — Shell tapering to an acute point, smooth; septa nume¬ 
rous, slightly arched on each side of an impressed line, which extends the whole length of 
the shell. There is usually about one-third of the shell where no marks of septa are visible. 
This fossil is usually much flattened, and often replaced by iron pyrites, which from de¬ 
composition prevents the septa from being visible. It is apparently the only species in the 
rock. 
Localities — Bloomfield, Le Roy, Avon, Seneca lake. 
2. Strophomena setigera, n. s. — Shell semi-oval, marked by numerous fine radiating striae, 
and a few concentric lines of growth; beak scarcely elevated; hinge line with six flexuous 
spines, which appear like bristles. The extent of the hinge line is proportionally much less 
than in S. cornuta of the Clinton group ; the striae are also coarser. 
N- B. The spines are too short and rigid in the figure, but there has been no time to have 
it reengraved. 
Locality — Avon. 
