288 BONY SPINES OF SHARKS. 
Fossil Spines J or Ichthyodorulites,^ 
The bony spines of the dorsal fins of the Port 
Jackson Shark (PI. 1. Fig. 18.) throw important 
Hght on the history of fossil Spines; and enable 
perishable nature of the cartilaginous bones to which they were 
attached ; hence the spines and teeth usually afford the only 
evidence of the former existence of these extinct fossil species. 
They are dispersed abundantly throughout all strata, from the 
Carboniferous series to the most recent Chalk. 
In Plate 27^, Figs. 1 , 2, represent a series of teeth of the 
genus Acrodus, in the family of Cestracionts, from the lias of 
Somersetshire; and PI. 27^, a series of teeth of the genus 
Ptychodus, in the same family, a genus which occurs abun- 
dantly and exclusively in the Chalk formation. 
In the section PI. 1, Fig. 19 represents a tooth of Psam- 
modus, and Fig. 19', a tooth of Orodus, from the Carboniferous 
limestone; and Fig. 18', a recent tooth of the Cestracion Phi- 
lippi. The Cestracion Philippi, (PI. 1, Fig. 18, and PI. 27^ A.) 
is the only living species in the family of Sharks that has flat 
tesselated teeth, and enables us to refer numerous fossil teeth of 
similar construction to the same family. As the small anterior 
cutting teeth (PI. 27*^, A. Figs. 1. 2. 5.) in this species, present a 
character of true Sharks, which has not been found in any of the 
fossil Cestracionts, we have in this dentition of a living species, 
the only known link that connects the nearly extinct family of 
Cestracionts with the true Sharks or Squaloids. 
The second division of the family of Sharks, Hybodonts, com- 
mencing probably with the Coal formation, prevailed during the 
deposition of all the Secondary strata beneath the Chalk ; the teeth 
of this division possess intermediate characters between the blunt 
polygonal crushing teeth of the sub-family Cestracion, and the 
smooth and sharp-edged cutting teeth of the Squaloids, or true 
Sharks, which commenced with the Cretaceous formations. They 
* See PI. 27^ C. 3. 
