FOSSIL FISHES. 53 



teeth, among which are species of Petalodus, Oladodus, and Gtenopty- 

 chit(,s. 



Of these the largest and most abundant belong to a species of Petalo- 

 dus, which seems to agree in all respects with P.Alleghaniensis, described 

 by Dr. Leidy, and first obtained from the Coal Measure limestones of 

 Pennsylvania. Two of these teeth are now figured, and they represent 

 fairly well the size and forms of the specimens found. It will be noticed 

 that in one of these the crown is higlier, and the root larger than in the 

 other. These differences are not constant, however, and can hardly have 

 specific value. They are probably due to the positions held by the differ- 

 ent teeth in the extended series which formed the dentition of the fish to 

 which they belonged. 



In the Report of the Geological Survey of Illinois, several species of 

 Petalodus are described, one of which, P. destructor, is from the Coal 

 Measures. From its large size and peculiar root, this tooth was consid- 

 ered distinct from P. Alley haniensis, but Mr. 0. St. John, who has given 

 much study to the fish teeth found in Illinois and Iowa, regards them as 

 identical. This seems to me probable, but it is not yet proved by any 

 facts which have come under ray observation. None of the specimens of 

 P. Alleghaniensis yet found in Ohio are more than half as large as some 

 of those from Sangamon County, or from Cassville, Illinois. They differ 

 too, in the form of the root ; the specimens from Ohio having more elon- 

 gated and narrower roots than the great teeth, with spatulate, pointed 

 roots, which are found in the Coal Measures of Illinois. These differences 

 may be only local, however, and it is quite possible that intermediate 

 forms will hereafter be found which shall connect the robust and power- 

 ful teeth of the West, with the smaller and more delicate forms which occur 

 in the Crinoidal limestone of Ohio. 



Ctewacawthus fokmosits, Newb. 



Plate LIX., Figs. 1-1«. 



This fine species of Ctenacanthus is described in Vol. I., Part II., p. 

 328, of this Eeport. The figure now given is taken from a very perfect 

 specimen obtained by M. C. Read, Esq., from the Cuyahoga shale at War- 

 ren, Trumbull County. It illustrates the size, form, and markings better 

 than the figure before given, and much better than any verbal description 

 could do. During the progress of the Survey, more or less perfect speci- 

 mens of this species were met with in various parts of the State, where the 

 Waverly rocks were exposed, and it is evident that the shark which bore 

 them was the most common, and probably the most formidable of those 



