52 PALEONTOLOGY OF NEW-YORK. 
CEPHALOPODA OF THE BLACK-RIVER LIMESTONE. 
Puates XIII. to XX. 
The Cepuatopopa of this period present a remarkable development in numbers of in- 
dividuals, though not a large number of species. The forms are all peculiar to the rock,* 
none of them having been found in the higher limestone, though other species of the same 
family are so largely developed during the Trenton limestone period. Some of the Or- 
THOCERATA attain the enormous length of eight or ten feet, and are not less than one foot in 
diameter. This period, though so well defined by its fossils, must have been one of short 
duration, during which the bottom of the ocean seems to have swarmed with myriads of 
these cephalopods. The deposition of calcareous matter which imbedded them, and, so far 
as known, destroyed the entire race, has not a thickness of above ten feet in New-York. 
Still the same deposit, containing the same species of fossils, is recognized on Lake Huron, 
and in Kentucky and Tennessee ; and even in Sweden, we have reason to believe that the 
same distinction may be made as here, and that in other parts of Europe the mass will be 
found marked by identical or representative species. 
86. 1. LITUITES UNDATUS. 
Pu. XIII. Figs. 1, 1 a, 6, & 3; and Pu. XIII. (bis). Fig. 1. 
Inachus undatus, Conrap in MS. Emmons, Geol. Report, pag. 394, fig. 1. 
Convolute, suborbicular ; spire equally depressed on either side ; volutions about four, 
contiguous, gradually increasing towards the aperture, which is subquadrate and scarcely 
expanded ; volutions rounded upon the sides and flattened upon the back, ventral side 
compressed from contact with the next volution ; surface undulated by strong oblique ridges 
and depressions, which rise from the inner side of the volutions, bend backwards, and 
become very strong upon the periphery ; entire surface covered with fine sub-imbricating 
striz, which are more distinct towards the aperture ; striz curving downwards, forming an 
arch upon the back of the shell. 
This rare and remarkable shell is readily distinguished by its flattened P riicnlar form, 
and strong retral ridges, which are very prominent on the outer edge of the volution, 
though usually scarcely indicated upon the plain flattened back of the last whorl, which is 
only marked by curved striz. 
* T have received a single species from Lewis county, as occurring in the Trenton limestone, which is identical with 
a species in this rock, and also the same from Herkimer county; but I am not quite satisfied of the position in these 
instances. 
