156 MOSTLY MAMMALS 



In spite of the fact that their early ancestors were provided 

 with a good working set of sharply pointed dental organs, 

 birds in these degenerate days manage to get along without 

 teeth at all. A few mammals, too, like the South American 

 ant-eaters, are in the same condition ; and some people have 

 thought that in a few more generations civilised man himself 

 will be reduced to the same toothless state. The great 

 majority of mammals, however, possess a more or less 

 efficient set of teeth, varying in shape, size, and number 

 according to the need of each particular species or group. 

 But there is one feature common to these organs in mammals 

 of all descriptions ; and this is that they are strictly confined 

 to the margins of the jaws, never extending either on to the 

 palate, or to the space enclosed between the two branches 

 of the lower jaw. In many reptiles, such as crocodiles and 

 a large number of lizards, the same law of dental arrange- 

 ment obtains. In some lizards, and still more markedly in 

 certain extinct members of the reptile class, we find, however, 

 a number of teeth developed on the palate, having flattened 

 crowns, and thus tending to make the mouth act the part of 

 one large millstone. But we must descend a stage farther 

 in the scale of animated nature before we come to structures 

 which are strictly comparable with artificial millstones and 

 crushing cylinders. And it is in the class of fishes that we 

 meet with these organs in the full perfection of this type of 

 development. Not that they occur by any means in all the 

 groups of that class ; the fact being that at the present day 

 living millstones are going out of fashion, the great pre- 

 ponderance of modern fishes having their dental armature 

 mainly restricted to the margin of the jaws, with or without 

 a minor development of crushing teeth on the palate or the 

 bones of the gullet. With the exception of a comparatively 

 limited number of cases, showing a different type of develop- 



