72 
GEOLOGY OF THE FOURTH DISTRICT. 
smaller; the mesial elevation of the upper valve also appears more prominent, judging from 
the figure. This shell occurs in the iron ore bed at Rochester. 
3. The Atrypa naviformis is a nearly globular smooth shell, the beak extending considerably, 
and incurved; the lower valve is much elevated in front, after the manner of A. linguifera 
and A. congesta. This shell is typical of the upper limestone of the group, and so far as I 
know, is found in no other rock. 
4. The Atrypa plicatula presents three of the folds raised in front, and from six to eight 
on either side ; three of them very distinct, the remaining ones obscure. There is a slight 
depression in the upper valve, extending from the beak one-third the length of the shell. 
5. Joints or plates of Actinocrinus ? plumosus. — This fossil is found in greater or less 
profusion in the lower limestone, throughout the extent of the district. It appears in great 
numbers one mile south of Medina, in the weathered specimens of the rock. Large surfaces 
are often covered with them, usually presenting the form given in the illustration. A mile 
west of Reynolds’ basin they are equally abundant, associated with the Atrypa congesta and 
A. plicatula. 
With the crinoidal joint are found some plumose tentaculce of a crinoid, which apparently 
belong to the same species. It is provisionally placed in the genus Actinocrinus, and this 
character of the tentaculse gives its specific name. 
In the green shale we find, for the most part, a different association of fossils from those 
of the limestones, though two or three species are common to both. In the accompanying 
illustration are presented the most common forms found in both the upper and lower green 
shale. 
17. 
1. Strophomena elegantula. 
2. S. corrugata. 
3. S. comuta. 
4. Atrypa hemispherica. 
5. Littorina cancellata (young shell.) 
6. Littorina cancellata (old shell) 
7. Posidonia? alata. 
8. Atrypa affinis ? var ? 
8 a. — — (young shell). 
9. Calymene? trisulcata. 
10. Agnostus latus, natural size and magnified. 
11. Tentaculites minutus, natural size and magnified. 
12. Graptolites Clintonensis, 
