GEOLOGY OF THE SECOND DISTRICT. 
the mouth; but in a single and much larger specimen, it appeared that the mouth was 
remarkably expanded and wide, so much so as to be apparently out of proportion to the rest 
of the animal. 
104. 
1. Inachus undatus. 2, edge view. 
This remarkable fossil is found at Watertown, in the black limestone. It is rare. Casts 
sometimes occur which arc smooth. 
The following bivalves may be considered as characteristic of the Trenton limestone: 
105. 
i 
1. Strophomcna sencea. 
Nos. 1 and 2, but more particularly the latter, abound at numerous localities in this county. 
No. d. This is probably as constant at all tlie localities in the trcnton rock which have been 
examined, as any fossil which I have hitherto observed. 
