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 



