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GEOLOGY OF THE FOURTH DISTRICT. 
In addition to the fossils already presented from this group, we might introduce many more, 
perhaps equally characteristic in different localities. The following among the Delthyris, may 
serve to illustrate the prevailing forms of this genus. 
1. Delthyris granulifera. — View of upper valve, presenting the duplicate mesial fold and 
the granulated surface. 
1 a. End view of the same, showing the curved beak of the lower valve, and the spiral coil 
within the shell. 
1 h. A specimen with a more extended hinge line. 
1 c, and 1 d. Two views of a smaller specimen of the same fossil. 
This is by no means an uncommon fossil, though it is usually much compressed and dis¬ 
torted, and the delicate points or granulations upon its surface are worn off. The specimen 
fig. 1 is very free from compression, and presents the parts in true proportion to each other. 
In fig. 1 b, the area is narrowed, from the upper valve having been pressed backward. The 
fossil is very variable in the proportionate extension of the hinge line, sometimes being pro¬ 
duced at the extremities into a point. 
This specimen is from Moscow, Livingston county. In the smaller figure, the concentric 
laminae are numerous and sharp, as is the fact in many species of this genus. 
2. Delthyris congesta. — View of the upper valve, showing the broad mesial fold, and broad 
plain ribs. 
2 a. Front view, showing the elevation of the lower valve into a linguiform extension, and 
the deep mesial sinus. 
This is a remarkably rotund fossil when pressed, but when compressed, often much re¬ 
sembles the last, and is frequently mistaken for it. The mesial fold, however, is plain, and 
the surface free from granulations; the ribs are likewise fewer in number. 
3. Delthyris macronota. — View of hinge, showing the broad area and narrow aperture. 
3 a. View of the area of the lower valve. 
3 b. Lower valve. 
This fossil is always readily distinguished by the broad area, the narrow aperture, and the 
numerous small plain ribs and strong concentric laminae of growth. It is more abundant at 
Moscow than in any other part of the district; though it is of frequent occurrence on the 
shores of Seneca lake, and in other places. 
