662 MAMMALIA. 



cular mesodermic dermis is defined off as the " dentine 

 germ." The crown of this papilla becomes hard, and the 

 ossification proceeds downwards and inwards, while above 

 the dentine crown the enamel begins to form a hard cap. 

 Meantime the tissue around the base of the tooth-papilla 

 becomes differentiated into an enclosing follicle or sac, 

 from the inner layer of which the cement is developed. 

 The pulp is but the uncalcified core of the papilla. 



The base of a tooth may remain unconstricted, and the core of pulp 

 may persist. Such a tooth goes on growing, its growth usually keeping 

 pace with the rate at which the apex is worn away with use, and it is 

 described as "rootless" and "with persistent pulp." The incisors of 

 Rodents and of elephants illustrate this condition. 



In the development of most teeth, however, the base is narrowed 

 and prolonged into a root or several roots which become firmly fixed 

 in the socket. Through a minute aperture at the end of the root, 

 blood vessels and nerves still enter the pulp-cavity and keep the tooth 

 alive, but, as the limit of growth is reached, the residue of soft pulp tends 

 to disappear. 



The two most marked characteristics of the teeth of Mammals are, 

 that they are typically heterodont, that is, different from one another in 

 form and function, and that the succession is practically reduced to 

 two sets, a condition described as diphyodont as contrasted with the 

 polyphyodont condition seen in Fishes and Reptiles, where the suc- 

 cession is practically unlimited. 



As exceptions, there are cases like that of the dolphins, where the 

 teeth are uniform or homodont and very numerous. This, however, 

 is not a primitive, but a secondarily acquired condition. 



In the typical dentition of Mammals there are forty-four 

 permanent teeth, eleven on each side above and below ; but 

 it is rare in the Eutherian Mammals to find the full number 

 developed, and the dentitions of the Marsupials, of the 

 Edentates, and of the Cetacea cannot be reduced to this 

 type. The eleven on each of the upper jaws may be divided 

 in the typical case into four sets. Most anteriorly, associated 

 with the premaxilla, are three simple, single-rooted teeth, 

 usually adapted for cutting or seizing. These are called 

 incisors. Posteriorly there are crushing or grinding teeth, 

 whose crowns bear cusps or cones, or are variously ridged, 

 and which have two or more roots associated with the 

 maxilla. But of these grinders the last three occur as one 

 set, having no calcified successors, or, as others maintain, 

 having no milk predecessors. They are therefore dis- 

 tinguished as true molars, from the four more anterior and 



