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 tlian 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 v^hich 

 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 

 cemeiit. 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 is a 

 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, i.e.^ dififer 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 incisors. 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). 



