FISHES OF THE CARBONIFEROUS SYSTEM. 185 
modern Rays. Another form of crushing teeth is that of Orodus, in which 
the crown rises in a series of hillocks, forming a miniature mountain chain 
(whence the name), of which the central summit is highest. Of these teeth 
there is a great variety. Some of them have the crown most elaborately 
carved and ornamented, and some are of great size; Orodus ramosus, of the 
Mountain limestone, having the mouth filled with a hundred or more teeth 
that were from two to five inches in length. A closely allied, if not identi- 
cal, species from the Carboniferous limestone of Illinois was still larger. 
A group of Sharks with peculiar cutting teeth—the Petalodontide— 
formed a conspicuous feature in the Carboniferous fish-fauna running 
through the Carboniferous limestone and the Coal Measures. These had 
teeth of which the crowns had the form of the blade of a long-handled 
shovel, and were usually attached to a strong root, that must have been 
firmly planted in the integuments of the jaw. Auntliodus is a form related to 
Petalodus, but in the teeth of this genus the root is very small or wanting. 
In Polyrhizodus the crown of the tooth was like that of Chomatodus, while 
the root was divided into a number of lobes. An allied form, Dactylodus, 
had the crown of Petalodus and a root consisting of many finger-like 
branches. 
The Sharks with piercing teeth formed the genus Cladodus and its allies, 
which were numerous and wide-spread during all the Carboniferous age. 
These had teeth with broad semilunar bases, which afforded a firm sup- 
port to an acute, conical, and usually ancipital central cone, flanked by 
one or more lateral denticles, of which the exterior pair were largest. Some 
of the species of Cladodus must have been large and formidable fishes; the 
teeth were in many rows, several hundred in each set, and the central cone 
was sometimes an inch and a half in length. 
The spines of the Carboniferous Sharks have been already alluded to; 
and they also afford proof of the size of their wearers. Many of the spines 
of Ctenacanthus attain the length of a foot or more, while the great spines of 
Phoderacanthus, described by Mr. J. W. Davis, were more than two feet in 
length and six inches in diameter at the base. 
The following isa list of the genera, with the number of species in each, 
up to the present time, described from the Lower Carboniferous limestone 
