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- 
