LIVING MILLSTONES 163 
have gained a true idea of the dental armature of their 
extinct relatives which abounded in the seas of the Jurassic 
epoch. Visitors to Whitby must be familiar with certain 
black oblong fossils of about an inch and a half in length 
known to the quarrymen as “ fossil leeches.” These are 
the hinder teeth of an extinct shark (Aséeracanthus) nearly 
allied to the Port Jackson species, but of much larger size ; 
and although they are more rugose than pitted, they show 
the same smooth line on the summit. A beautiful specimen 
from Caen, in the British Museum, shows that the arrange- 
ment of these hinder teeth was almost exactly the same 
as in Cestvacion, which may thus be regarded as a survivor 
from a long-past epoch of the earth’s history. 
But there were other “ millstone-mouthed” sharks at a 
still earlier period which appear to have been allied to 
Cestracion, although the degree of relationship is uncertain. 
In these Palaeozoic sharks, as exemplified by Cochlzodus, 
the series of hinder teeth seems to have had an arrangement 
very similar to that obtaining in Cestracion, but the indi- 
vidual teeth of several series were more or less completely 
fused into a single solid plate, the ridges on which mark 
the original lines of division between the component series. 
These sharks exhibit, therefore, one among many instances 
where the earlier forms of a group are in some respects 
more specialised than their descendants, 
So much space has been taken up by the rays and sharks 
that only a few lines remain for the millstones of the enamel- 
scaled fishes. In none of these do the teeth, which are 
developed on most of the bones of both the upper and lower 
jaws, ever form continuous plates; and they are generally 
either spherical or kidney-bean-shaped and arranged in more 
or less longitudinal rows. Unlike those of the sharks and 
rays, these teeth, as in the familiar Lepzdotus of the Wealden, 
