THREE SUB-FAMILIES. 287 
tained to be dorsal spines of Fishes, and from 
their supposed defensive office, like those of the 
genus Balistes and Sihirus, have been named 
Ichthyodoruhtes. 
M. Agassiz has at length referred all these 
bodies to extinct genera in the great family of 
Sharks, a family which he separates into three 
sub-families, each containing forms peculiar to 
certain geological epochs, and which change 
simultaneously with the other great changes in 
fossil remains. 
The first and oldest sub-family, Cestra- 
cionts, beginning with the Transition strata, 
appears in every subsequent formation, till the 
commencement of the Tertiary, and has only 
one living representative, viz. the Cestracion 
Philippi, or Port Jackson Shark. (PL 1. Fig. 
18.) The second family, Hyhodonts, beginning 
with the Muschel-kalk, and perhaps with the 
Coal formation, prevails throughout the Oolite 
series, and ceases at the commencement of the 
Chalk. The third family of " Squaloids,'' or 
true Sharks, commences with the Cretaceous 
formation, and extends through the Tertiary 
strata into the actual creation.* 
* The character of the Cestracionts is marked by the presence 
of large polygonal obtuse enamelled teeth, covering the interior 
of the mouth with a kind of tesselated pavement. (PL 27*^. A. 1, 
3, 4, and PI. 27^, B. 1,2, 3, 4, 5.) In some species not less 
than sixty of these teeth occupied each jaw. They are rarely 
found connected together in a fossil state, in consequence of the 
