FOSSIL FISHES. 49 



Sniallef form without lateral denticles, from 2 to 3 lines high, central 

 cone and base as in larger form. These small teeth occur in large num- 

 l)ers in tlie fish bed discovered by Mr. Jay Terrell in the valley of Black 

 River. As will be seen from the description given above, they differ 

 mncli among themselves in form and size, bnt between the extremes are 

 all possible shades of variation. The peculiar yoke-shaped and cnpped 

 base is the same in all, but some have no lateral denticles, others have 

 tubercles at the sides of the central cone, while others still have two well- 

 developed lateral cones. This diversity is not greater than we find in 

 the dentition of modern sharks, and it is highly probable that in all of 

 the species of Cladodus, the teeth were more or less variable in size 

 and form according to the places they held in the month. This intro 

 duces an element of uncertainty in the diagnosis of species from a few 

 teeth, which should be kept in mind in describing or discussing species 

 of this and other genera of extinct sharks. It is not probable, however, 

 that the species of Cladodus have been very mnch multiplied from this 

 cause, for the reasons that : (1) the number of species yet described is 

 small, (2) tliey are distributed throngh a great vertical and geographical 

 range — from the base of the Devonian to the Permian, and from Russia 

 to Kansas — very few having been taken from the same stratum and 

 locality, and (3) the dentition of each species, however much varied, has 

 generally something in common, of form or ornamentation, by which the 

 different members of the series may, with due care, be identified. 



The danger of mnltiplying species from this canse is at least not suffi- 

 cient to make it necessary or wise to neglect all material of this kind 

 until snch time as the complete dentition of each species shall be discov- 

 ered. This cannot be hoped for in regard to many fossil sharks, as their 

 skeletons were cartilaginons, and the connection between the teeth and 

 jaws was ligamentous and was dissolved in decay. In the only cases that 

 have come under my observation where any consideraljle portion of the 

 dental series has been found with the teeth in position {Cladodus Patier- 

 soni and Diplodus comjrressns), the only difference visible among some 

 hundr.eds of teeth shown is in size. Had the entire dentition of both 

 jaws been visible in these cases, their testimony would have been more 

 conclusive, but it lias much weight as it is. 



Cladodus Romingeiu (n. sp.). 



Teeth small, breadth of base 7 lines, height of median cone 5 lines ; 

 central cone flattened behind, anterior face rounded ; basal portion of 

 posterior face deeply sinused ; lateral cones two pairs (sometimes with a 



