as indicative of the Age of the Animal. 279 
planted by roots or fan<;s into distinct sockets in the jaws. When 
attached to the bones beneath the membrane, as they are some- 
times found to be, they are so attached by distinct ossification of 
their expanded bases. Occasionally these teeth are fcAV in num- 
ber, but often they are so numerous as to thickly stud the greater 
part of the surface of the mouth. As in the mammalian class, 
the teeth of fishes present many varieties of form, each being 
suited to the kind of food on which these creatures live. In 
most fishes the teeth are shed and renewed several times 
during life. In this respect fishes differ greatly from mammals, 
as in them there are but two sets, the temporary and the perma- 
nent. If torn away, fresh teeth soon arise from the membrane of 
the mouth of the fish. 
Their formation may be thus briefly described : 
Projections or j)ajnU(v spring from the membrane, they become 
very vascular, increase in size, secrete a layer of osseous matter 
upon their free surfaces, and are thus converted into true teeth. 
This plan is only slightly modified in the formation of the teeth 
of mammals. These are likewise produced from papillae which 
arise from the mucous membrane, and hav^ at first no connexion 
with the bones. The several stages of their development will 
hereafter be entered upon. It is svifhcient therefore merely to 
allude to the fact in this place, in order to show the close identity 
of the two processes. 
Many other illustrations in various animals, showing that the 
teeth belong to the mucous membrane as an internal skin, and not 
to the skeleton, or bony parts of the frame, might be given. Such, 
however, are not now required. It must, nevertheless, be stated in 
explanation that the membrane which lines the digestive and the 
other canals of the body is but a modified form of tiie external skin. 
Vascular and sensitive papillae are developed on both, and these 
are protected in either situation by a dense and insensible epider- 
moid or cuticular covering. The sides of the mouth of the ox and 
sheep afford striking examples of papilla* growing from a mucous 
membrane. Long conical shaped papillae thickly beset these 
parts, and also the tongue of the ox ; having here a covering so 
dense as to approach the character of soft horn. On the tongues 
of the feline tribe the papillae have true horny sheaths. In the 
mouths of fishes they have bony, or modified bony coverings, 
becoming thereby, as before explained, the teeth of these crea- 
tures. These gradations are few and easily understood. 
Thus we see that teeth, whether situated in the stomach, as 
in the Crustacea ; in the gullet, as in fishes ; or in bony sockets, 
as in the jaws of the higher order of animals, are essentially the 
same in belonging to the membranous parts of the body. The 
practical bearing of these prefatory observations will be ren- 
