222 
GEOLOGY OF THE FOURTH DISTRICT, 
they are more rarely found, and finally do not appear at all. The Aviculee, on the other hand, 
appear to increase in numbers in a westerly direction, and on Lake Erie are more numerous 
than at any other point. 
With the termination of the Moscow shale, seem to have perished the immense number of 
organic forms which every where mark the presence of that rock. For a long period, as is 
evident from the character of this shale, the ocean bed was not inhabited by living things ; and 
with the exception of the few species in the Tully limestone, where that rock occurs, it is not 
until we arrive at nearly the termination of this slate, that vitality again assumes its place 
among the laws of nature, to go on increasing, till in the upper part of the Chemung group 
the ocean literally swarmed with living things. 
94, 
1 and 2. Avicula fragilis. 3. Strophomena setigera. 4. Tentaculites Cssurella, 
1 and 2. Avicula fragilis , n. s. — Shell ovate, (hinge line being usually concealed,) very 
thin; surface marked by concentric lines, and a few faint radiating striae ; hinge line shorter 
than the width of the shell. 
Fig. 1 is a small fragment of the shale, with these shells covering the surface. 
Fig. 2, a single shell, showing the perfect form. 
It is an extremely abundant fossil, but from being mutilated, and often partially concealed, 
its generic relations are not always readily detected. It occurs in the Marcellus shale. 
Localities —Seneca county ; Geneseo ; Leicester ; Mount-Morris ; Eighteen-mile creek. 
3. Strophomena setigera (see page 180 of this Report). This fossil is also common to the 
Marcellus shale. 
Locality —Outlet of Crooked lake. 
4. Tentaculites fissurella. A fragment of the shale covered with the fossil (see page 180 of 
this Report). Occurs likewise in the Marcellus shale. 
The three species here figured are all common to this rock and the Marcellus shale, the 
lithological character of the two being precisely similar. Neither of them have been noticed 
in the intervening rocks of the Hamilton group, though the Avicula passes upwards into the 
Cashaqua shale of the Portage group. This is an interesting example of the recurrence of the 
same species of fossils in rocks widely separated from each other. This identity in fossils, 
and similarity in lithological characters, has been the cause of frequent mistakes in identifying 
the two. 
