158 MOSTLY MAMMALS 
important to notice that there has been an even more 
strongly marked tendency to the extinction of the enamel- 
scaled fishes, and their replacement by the ordinary soft- 
scaled fishes so abundant in the present seas. As the 
majority of these old mail-clad fishes, as well as a large 
proportion of the ancient sharks, were provided with 
crushing teeth, it is a fair inference that their food con- 
sisted largely of shell-fish and crustaceans, with a certain 
proportion of their own mail-clad relatives. When, how- 
ever, the swift-swimming, soft-scaled fishes came to the 
fore, they would naturally offer a more tempting and 
nourishing diet to such sharks and other predaceous 
members of their own class as were swift enough in their 
movements to make them their prey. And consequently 
the old millstone-jawed sharks would tend more or less 
completely to disappear. On the other hand, the skates and 
rays, which are for the most part slow-moving creatures, 
flapping sluggishly along on the sea-bottom by means of 
their fan-like fins, would be quite unable to capture the 
modern type of swift-swimming fish. And they have thus 
had to content themselves with the old-fashioned diet of 
shell-fish and crabs, in consequence of which a large pro- 
portion of them have retained the dental millstones which 
have been so steadily going out of fashion among their 
more advanced relatives. Not that these rays and skates 
have by any means been content with the kind of molar 
machinery that did duty for their forefathers, since some 
of them, together with their Tertiary ancestors, have de- 
veloped what appears to be an absolutely perfect type of 
living mill, far superior to that which served the purpose 
of their predecessors. And it must always be remembered 
that these beautiful living millstones and cylinders (which 
are some of the most exquisite bony structures to be met 
