COMPARATIVE ANATOMY OF TEETH. 
81 
and, like the hair and nails, are appendages of the skin. The 
teeth of the shark, and of many other creatures, remain 
imbedded in tough mucous membrane, and never acquire any 
connection with the bone. Whether teeth have a bony or 
tegumentary connection with the skeleton lias been much 
discussed, and it may be well here to show some grounds for 
the belief in their connection with and development from the 
skin. If a transverse section through the jaw of a dogfish 
(Scyllium ccinicula) be examined, we shall find that the forming 
teeth lie upon the inside of the semi-ossified jawbones, the 
youngest being at the bottom; progressing upwards each 
tooth is more fully calcified till on passing over the border of 
the jaw we come to those teeth whose period of greatest 
usefulness is past, and which are about to be cast off, in the 
course of that slow rotation of the whole tooth - bearing 
mucous membrane over the border of the jaw which is 
constantly going on. The dentine germs and consequently 
the dentine are indisputably derived from the connective 
tissue of the mucous membrane immediately subjacent to the 
epithelium, nor can it be doubted that the enamel organs are 
simply the modified epithelium of that same mucous 
membrane. The teeth of man and other mammalia being 
set in bony sockets has given rise to the opinion that they 
were developed in the bone, whereas their germs are to be 
found in the mucous membrane and the subjacent tissue, and 
the bony sockets grow around the forming teeth ; or as in the 
case of the sharks just quoted there is no bony socket, but 
simply a membranous attachment. Besides this, the teeth 
begin to be formed when there is no bone at all. 
The attachment of teeth in the various animals is in itself 
a very interesting study. Although the gradations from one 
class to another prevent any absolutely correct classification, 
four principal methods may be enumerated, viz., attachment 
by means of fibrous membrane, by a hinge, by anchylosis, and 
by implantation in bony sockets. The fibrous attachment I 
have already alluded to in the shark. In this animal the 
teeth formed inside the jaw rise gradually to its crest 
and then work round to the outside and drop into the sea. 
This would appear to account for the great numbers of fossil 
sharks’ teeth to be found. Then there is attachment by an 
elastic hinge. The possession of moveable teeth able to yield 
to pressure and subsequently to resume the upright position 
was formerly supposed to be confined to the lophius (angler) 
and its immediate allies. They have, however, been found 
in the common pike (esox) and in the gadidae (cod tribe), so 
that as they occur in these fish, so widely removed from one 
another in other respects, it is probable that further investi- 
