116 
GEOLOGY OF THE FOURTH DISTRICT. 
43 . 
1. - ? This fossil is completely flattened, presenting no solid substance, except a 
thin carbonaceous film. It appears like a collection of fine hairs arranged obliquely on a 
central axis like the tail of some animal. It resembles in structure some of the solid corals 
where the pores are oblique to the direction of the axis, but here there is no defined margin, 
and the calcareous matter, if ever existing, has been removed. 
2, 3. Coral resembling Isis. This fossil has a calcareous stem and the external structure 
is much like some of the recent corals. Figure 2 represents the natural size, and fig. 3 is a 
portion magnified. It is the only coral presenting this structure which I have met with in the 
New-York rocks. It occurs at Lockport, with the last, in the shale of this group. 
Since there are numerous corals in this shale still retaining their calcareous character and 
some of the expanded or net-like forms, which are always clearly referable to structures of 
this kind, it seems natural to infer that those forms where only a thin carbonaceous film 
remains, with no visible pores, or structure like the stony corals, should be referred to some 
other division of the Polyparia at least, and perhaps even to some of the more gelatinous 
or less solid forms than Gorgonia. 
The fossils figured in the preceding wood cuts with the exception of two or three species, 
are, so far as known, confined to this group. From the identity of many of these with forms 
described by Mr. Murchison, from the Wenlock shale, and limestone of England, we have 
inferred that the Niagara group of New-York represents, to a considerable degree, that for¬ 
mation. Though the Wenlock limestone contains some forms identical with those of the 
