LIVING MILLSTONES 157 
ment, to which it is not my present intention to allude, these 
dental millstones are confined at the present day to those 
hideous marine fishes commonly known as skates and rays, 
and to the singular Port Jackson shark and a few allied 
species inhabiting the Pacific and Malayan seas. On the 
other hand, the seas of the Cretaceous, Jurassic, and ante- 
cedent epochs absolutely swarmed with numerous kinds of 
sharks more or less nearly related to the Port Jackson 
species, whose mouths were filled with pavements of teeth 
showing marvellous variety of structure and beauty of 
ornamentation. The skates and rays, too, displayed types 
of dental millstones quite unlike any of those of the present 
day. And in addition to these, there were hosts of enamel- 
scaled fishes whose mouths were likewise crammed with 
beautiful crushing teeth, albeit of a totally different type 
of structure to that obtaining in either the sharks or the 
rays. Although well-nigh extinct, these enamel-scaled 
fishes are still represented by the bony pike of the rivers 
of North America and the bichir (remarkable for its fringed 
fins and the row of finlets down its back) of tropical 
Africa. But it is noteworthy that in neither of these sur- 
vivors of an ancient group do we find the mouth furnished 
with an apparatus of millstones; while, as already said, 
among the’ host of sharks that infest the warmer seas of 
the globe it is only in the Port Jackson species and its three 
kindred that we find similar structures retained; all the 
other members of the group having developed cuspidate 
teeth adapted for seizing and tearing soft-fleshed prey, 
instead of for grinding-up mail-clad food. 
Clearly, then, there has been some general cause at work 
which has rendered crushing teeth, so to speak, unfashion- 
able among the fishes of the present day and the imme- 
diately antecedent epochs. And in this connection it is 
