Glass 6. Mammalia. 



485 



very poorly developed, or even rudimentary. The molars are usually the 

 most complicated teeoh, whilst the incisors and canines are simpler ; 

 the canines are generally conical, the incisors, for the most part, 

 chisel-shaped. The teeth of the milk dentition usually resemble those 

 of the permanent set ; but a given milk tooth is not always exactly 

 like that which is to take its place : in the Carnivora, for instance, 

 each milk molar is very like the permanent molar, one place further 

 back in the series. In some placental Mammals, there may be 

 conditions very different from those just described, in that a larger 

 number of teeth may be present. This is especially the case in 

 forms which, in correlation with peculiar habits, have in some 

 respects, descended, so to speak, to a lower zoological grade; for 

 instance, in the Toothed Whales, whose mode of life closely resembles 

 that of Fish, the teeth are uniform, usually conical in shape (homodont), 

 and very numerous ; in animals, too, whose teeth are of subordinate 

 importance, there may be an increase in number accompanying a 

 degeneration in form and structure {e.g., in the Dasypodidse) . Where 

 there is so aberrant a condition of the permanent teeth, there 

 is frequently an entire absence of milk dentition. The permanent 

 dentition of the Marsupials differs from that of the Placentalia in 

 that it is composed of a larger number of teeth, and also, that the 

 milk dentition is represented by a single molar ; for details see this 

 group. 



The following points may be added to the description of the mammalian 

 dentition just given. The enamel is frequently thinner in some parts of the 



Pig. 395. jilncisor of a Dog, shortly 

 after it has come into use ; B the same 

 tooth of an old Dog ; longitudinal 

 section. In the young tooth the pulp 

 cavity is very large ; there is as yet no 

 cement (or very little) ; in the old tooth 

 the upper portion of the primitive pulp 

 cavity is entirely filled with dentine, and 

 the rest is very narrow, the cement 

 abundant, the tip of the tooth worn 

 away, c cement, d dentine, p pulp cavity, 

 s enamel. — Orig. 



crown than in others ; or it may be absent from certain regions {e.g., from the 

 posterior side of the incisors of the Rodents) ; from almost the whole tooth (in the 

 incisors of Elephants, enamel occm's only upon the tip of the tooth before it is 

 out) ; or it may be completely wanting (as in many Whales). When the tooth is 

 cut and comes into use, it is not, as a rule, completely developed; the root 

 is frequently not yet fully formed; the dentine has not attained its greatest 

 thickness ; the pulp-cavity is large, and decreases gradually as the bulk of the 

 dentine increases; the cement at the root of the tooth also continues to be 

 deposited, and in very old animals, is often of considerable thickness ; whilst in 



