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 



