396 
GEOLOGY OF THE SECOND DISTRICT. 
No. 1. Ortliis lept(Bnoides, belongs to the grey variety of the trenton rock; but judging 
from the small number yet found, it ought to be set down as rare. 
No. 2. Orthoceratites trentonensis, is common ; and what is rather singular, we meet only 
with this portion of a part which is slightly curved. 
No. 3. Orthoceratite, is also common, and belongs to the grey variety of the limestone. 
No. 4. Ortliis hisulcata, is one of the smallest of this genus which has been seen in the 
trenton rock. It was found in Adams, where it is quite common, b, natural size ; a, enlarged. 
Nos. 5 and 6. Both species are quite common ; the first is found in the black limestone at 
Watertown ; the other in the grey, south of the village, and has a wide distribution. I 
observed it more or less perfect at all the localities which were examined. It is, however, 
always a cast, and I have never been able to procure the fossil entire. In stating the position 
in which the fossil has been found, it is not for the purpose of conveying an impression that 
the two varieties are distinct masses. The fossils will not be found confined to either; and 
furthermore, they alternate with each other. Still, in several instances, so far as observations 
have been made, some fossils appear more abundant in one than in the other. The black 
variety is a limestone less pure than the grey : it is generally interlaminated with shale, and 
hence its composition is adapted to some species of animals, and much less so to others; 
and so also the same remark applies to the grey limestone. 
The Delthyris -occurs in the grey limestone at Watertown, and 
the Delthyris expansus (No. 2, Fig. 109) is associated with it; the)’’ are 
obtained only in casts, and both are rare. 
The trenton appears once more to limit a genus, so far as the rocks of 
the northern district are concerned : this genus does not occur below. There are some unex¬ 
plored places Avhere it is possible the Delthyris and the Pterinea may be discovered. I refer 
to the upper part of the calciferous sandrock, which has furnished already several new species. 
