124 
GEOLOGY OF THE THIRD DISTRICT. 
the Potsdam sandstone, are the only two rocks of the district which present unaltered the sand 
of the primary region, as it appears where pure. 
The localities of the sandstone are more numerous at the west end of the district, than at 
the east end. 
The fossil shells in this sandstone are of an unusually large size, and in most localities 
crowded together, and near the lower part of the rock. The most common and characteristic 
ones in the district are Nos. 1 and 2 of the preceding wood-cut, and No. 4 of the succeeding 
one, which represents the internal form or cavity of the Atrypa unguiformis of Mr. Conrad. 
This truly remarkable impression in no small degree resembles the under part of the hoof of 
a colt, and is often taken for such. 
This fossil cannot, consistently with what is known of the genus Atrypa, belong to it. It 
stands alone ? but its association with two other casts in the same sandstone, and a fourth 
in the Onondaga limestone, shows it to belong to a genus between Strophomena, Orthis and 
Atrypa, uniting probably the three. A good name at present for the genus, will be Hippa¬ 
rionix, the right meaning of which is Colt's nail, but by which we mean hoof; it being 
a singular fact that neither the Greek or Latin language has any term for hoof, as in the 
Saxon ; and when it shall be desirable to give precision of language to natural science, the 
three languages must be united. 
There being but three species that we know belong to this genus, their names in the order 
of resemblance, will be Proximate hipparionix (H. proximus), Consimilar hipparionix (H. 
consimilis), and Similar hipparionix (H. similaris). 
29. 
No. 4. Cast of internal part of the Proximate hipparionix. 
No. 5. Cast of the internal part of the Arenaceous delthyris. This latter cast is very abundant, and its peculiar appearance 
has given rise to many fanciful resemblances. 
