396 



GEOLOGY OF THE SECOND DISTRICT. 



107. 



1. Orlhis leptasnoides. 



2. Orlhoceratitea trentonensis. 



3. Orthoceratite. 



4. Orthis bisulcata. 



5. Pleurotomaria. 



6. Pleurotomaria. 



No. 1. Orthis Icpteenoides, belongs to the grey variety of the trcnton 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. Orthis bisulcata, is one of the smallest of this genus which has been seen in the 

 trenton rock. It was found in Adams, wliere it is quite common, b, natural size ; «, enlarged. 



Nos. 5 and 0. Both species are quite common ; the first is found in the black limestone at 

 Watertown ; tlic 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 Ddthyris occurs in the grey limestone at Watertown, and 



the Dehhijris expansus (No. 2, Fig. 109) is associated with it; they 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 where 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. 



