ICHTHYODORULITES. 289 
US to refer those very common, but little under- 
stood fossils, which have been called Ichthyodo- 
rulites, to extinct genera and species of the sub- 
family of Cestracionts. (See page 286). Several 
living species of the great family of Sharks have 
are distinguished from those of true Sharks by being plicated, both 
on the external and internal surface of the enamel. (See Plate 
27*^. B. Figs. 8, 9, 10). Plate 27'' C. l.'""^ represents a rare 
example of a series of teeth of Hybodus reticulatus, still adhering 
to the cartilaginous jaw bones, from the Lias of Lyme Regis. 
Striated teeth of this family abound in the Stonesfield slate and 
in the Wealden formation. 
Another genus in the sub-family of Hybodonts, is the Onchus, 
found in the Lias at Lyme Regis ; the teeth of this genus are 
represented, PI. 27*^. B. 6, 7. 
In the third, or Squaloid division of fossils of this family, we 
have the character of true Sharks ; these appear for the first 
time in the Cretaceous formations, and extend through all the 
Tertiary deposits to the present era. (PI. 21^. B. 11, 12, 13.) 
In this division the surface of the teeth is always smooth on the 
outer side, and sometimes plicated on the inner side, as it is also 
in certain living species ; the teeth are often flat and lancet- 
shaped, with a sharp cutting border, which, in many species, is 
serrated with minute teeth. Species of this Squaloid family 
alone, abound in all strata of the Tertiary formation. 
The greater strength, and flattened condition of the teeth of 
the families of Sharks (Cestracionts and Hybodonts), that pre- 
vailed in the Transition and Secondary formations beneath the 
Chalk, had relation, most probably, to their office of crushing 
the hard coverings of the Crustacea, and of the bony enamelled 
scales of the Fishes, which formed their food. As soon as Fishes 
of the Cretaceous and Tertiary formations assumed the softer 
scales of modern Fishes, the teeth of the Squaloid sub-family 
assumed the sharp and cutting edges that characterise the teeth 
of living Sharks. Not one species of the blunt-toothed Cestra- 
ciont family has yet been discovered in any Tertiary formation, 
GEOL. u 
