MAMMALIA. 459 — 
dermis, commences to be modified by calcification into a hard dense 
enamel, whilst a layer of odontoblasts or cells of the dermis becomes 
active and gives rise on its outer side, near the enamel, to a bony 
dentine less dense than the enamel. In the centre the formation of 
dentine does not take place, so that a pulp-cavity remains. In the great 
majority of teeth this cavity becomes constricted and nearly closed and 
no further production of tooth-substance takes place; but in teeth which 
grow from persistent pulps, or continue to grow throughout life, the 
pulp-cavity remains widely open and the enamel-germ and odonto- 
blasts continue to produce fresh enamel and dentine respectively. 
To the teeth of many mammals is added a third substance called 
cement. This surrounds the dentine at the base of the tooth or lies 
between the enamel-crests on the upper surface of the tooth. It is 
produced by the dermis. As development proceeds the tooth forces 
its way to the surface and later its base becomes surrounded by bone, 
forming the socket. 
In most flesh-eating animals the enamel remains intact throughout life, 
but in vegetable-eaters the crown of the tooth, especially in the case of the 
molars, becomes worn away, and as the cement and dentine wear more 
rapidly than the enamel, the latter forms ridges which assist in mastica- 
tion. We may note in ‘this typical development of a tooth that it isa 
joint production of epidermis and dermis. The development is in 
essential features similar to that of a placoid scale (Elasmobranch 
fishes) and it is usual to regard the two structures as homologous. 
In the great majority of mammals the teeth are hetero- 
dont, z.e., differ markedly in shape and size in the different 
parts of the jaws. It is found impossible to directly com- 
pare the teeth of the same shape throughout the class as 
this would be a very unnatural grouping and would lead to 
confusion. For the determination of dental homologies we 
have to resort to other means. In the upper jaw the teeth 
are borne upon premaxilla in front and maxilla behind. All 
the teeth borne upon the former are called zxcisors. This 
name is given to them because, as a rule, they are chisel- 
shaped. They may, however, be of a very different shape, 
and their homology depends not upon their shape but upon 
their position on the premaxilla. The tooth immediately 
behind the suture between premaxilla and maxilla is known 
as the canine tooth because it is typically developed in dogs. 
It is usually a long single-rooted fang, but is often absent 
or of a different shape. The remainder of the teeth on the 
maxilla are called molars because they are mostly for grind- 
ing or cutting food ; they are usually many-cusped and have 
several roots. Some of the molars are further distinguished 
from the rest as premolars (see next page). 
