52 PAOWNTOLOOT OF NBW-TORK. 



CEPHALOPODA OF THE BLACK-RIVER LIMESTONE. 

 Plates Xm. to XX. 



The Cephalopoda 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 Oh- 

 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 cephjJopods. 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 ; eind 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 b© 

 found marked by identical or representative species. 



86. 1. LITUITES UNDATUS. 



Pl. Xni. Figs. 1, 1 a. 6, fc 3 ; and Pt. XIII. (bis). Fig. 1. 

 Inackiu tmdatuf, Conbad 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 

 striae, which are more distinct towards the aperture ; striee curving downwards, forming an 

 arch upon the back of the shell. 



This rare and remarkable shell is readily distinguished by its flattened orbicular 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 

 onlv marked by curved striae. 



* I have received a single species from Lewis county, as occurring in the Trenton limestone, which is identical with 

 * ipacie* in this rock, and also the same from Herkimer county ; but I am not quite satisfied of the position in these 

 instances. 



