HAMILTON GROUP. 
205 
84. 
I 
1. Dipleura Dekayi. 2 Orthonata undulata. 3. Delthyris mucronata. 
1. Head of Dipleura Dekayi (Green, Monograph, p. 79). — This portion of the fossil, 
as well as the caudal extremity, are usually found detached, and in the sandy shales of the 
Third District are abundant. In the softer shales of the Fourth District it is comparatively 
rare, though I have seen it in many places. 
It ranges from the Marcellus shale throughout the group, and I have found the caudal ex¬ 
tremity of a trilobite in the rocks of the Chemung group, apparently referable to the same 
species. 
2. Orthonata undulata (Conrad, Annual Reports ). — This fossil is not unfrequent on the 
eastern margin of the district, but far from presenting the perfectly delineated forms which it 
does farther east. It entirely disappears before reaching the Genesee valley. 
This and the last named fossil, which are typical of the group in the eastern part of the 
State, cannot be so considered throughout that portion west of Seneca lake. 
3. Delthyris mucronata (Conrad, Annual Reports). — This is the common form of this 
fossil in the sandy shales and shaly sandstones of the Third District; while, as we progress 
westward into the soft calcareous mudstones of the group, its hinge line becomes less extended, 
and the general form more rotund (see illustration, page 198 of this Report). It is one of the 
most numerous forms, and appears almost equally abundant throughout the length of the State, 
its maximum in the Fourth District being on the shore of Lake Erie. 
The other forms figured on page 152 in the Report of the Third District, viz. Orthoceras 
constrictum , Cypricardia recurva, Avicula flabella and Orbicula grandis, have all been seen 
in the Fourth District, but they are not considered among the typical fossils of this group. 
