56 
DAVIS : FOSSIL FISH EEMAINS. 
others as based on the type of the modern Cestracion or Port 
Jackson shark, differing from it, however, in possessing- few and 
large teeth in place of a greater number of small ones. Prof Agassiz 
in the third volume of his Poisson's Fossiles, p. 113, regards 
Cochliodus as ne<^irly related to Ceratodus ; and considers that the 
relationship proves that Ceratodus is a plagiostomous shark, both 
being closely related to the genus Cestracion. Since Prof. Agassiz 
penned his desciiption, an existing representative of the Ceratodus 
has been discovered in the mud-fish of /Vustralia, whose dentition 
is in every respect similar to the fossils described by Prof. Agassiz. 
The relationship of the living fish is proved to be with the Ganoids, 
and it is not improbable that the Cochliodonts of the Limestone 
Formations may eventually be found to be much more closely 
related to the Ceratodus than to the Cestracionts. The dentition 
of Orodus and the Amei'ican genus Agassizodus have been shewn 
to be very closely related to the living Cestracion ; they were 
co-existent with the Cochliodonts, and it is extremely improbable 
that they were closely related ; in arrangement and adaptation for 
feeding they are quite different from each other, the Cochliodonts 
for vegetable feeding and the Cestracionts to prey on Mollusca, &c. 
The teeth of the Pleurodus Ag. occur in the Limestone in 
Wensleydale with considerable frequence. They are small, and in 
all probability are a different species to those found in the coal 
measures. Though this genus possessed a cartilaginous skeleton, 
its remains have been discovered in the shales of the coal measures 
at Newsham in Northumberland, and in those of the West Riding 
of Yorkshire, so well preserved that the form of the fish could 
be ascertained. It was four inches to a foot in length, having a 
deeply rounded and flat body with a spine in front of the dorsal 
fin immediately behind the occiput about one third the length of 
the body, one half the length of the spine was probably buried in 
the integuments of the fish. In the Newsham specimen the 
position of the spine and the form of the fish are clearly indicated 
by the expanse of chagrin or dermal tubercles which has remained 
