70 
GEOLOGY OF THE FOURTH DISTRICT. 
Pentamerus oblongus. — Fig. lisa view of the larger valve of a full grown individual, 
showing a great expansion toward the base, and a tendency to a three-lobed character. The 
concentric lines of growth are strongly marked. 
Fig. 2 is the smaller valve of a younger shell, which presents a more circular form, but 
still the same tendency to trilobation. 
Fig. 3 is a side view of a perfect specimen, showing an intermediate proportion between 
the others. 
Figs. 4 and 5 are casts of the upper and lower valve, showing those divisions upon which 
the generic character depends. 
The Pentamerus commences in the Third District, and from its prominent characters fur¬ 
nishes a good guide to the existence of the group as far west as the Genesee river. It some¬ 
times occurs in the iron ore both in the Third and Fourth Districts. 
This fossil is typical of the calcareous beds in the Caradoc sandstone of England, and 
specimens of the mass with broken shells from the Hollies, can scarcely be distinguished 
from those on the Genesee and in Wayne county. Mr. Murchison has described two species, 
the P. lavis and P. oblongus, both of which appear to be represented in our numerous varie¬ 
ties, and are apparently referable to the same shell. 
1-5. Pentamerus oblongus. 
6. Delthyris brachynota. 
