IV. VERTEBRATA: MAMMALIA 



553 



in the jaws, and in those cases where continuous growth is necessary the 

 pulp persists and the teeth (incisors of rodents, tusks of elephants and pigs) 

 grow indefinitely. (3) In consequence of their greater hardness the 

 teeth are not used up so fast and do not require rapid replacement. There 

 occurs at most only one change, in which the dentition present at birth 

 or developed soon after — the milk, or lacteal, dentition or, better, first 

 dentition — is replaced by the second or permanent dentition (diphyodont 

 mammals). In some cases {monopliyodont mammals) there is no change, 

 the first dentition being permanently retained (marsupials, perhaps 

 toothed whales), or the first dentition is more or less rudimentary 

 (edentates, many rodents, bats, seals, some insectivores) . Besides the 

 two typical dentitions traces of a third or even a fourth may occur. A 

 prelacteal dentition of calcified germs which are never functional is best 

 seen in marsupials, and is rare in placental mammals. A post-permanent 

 dentition follo^\ing the permanent one is outlined in many placentalia, 

 and some of its teeth may exceptionally come into function. (4) Among 

 the teeth a division of labor has brought 

 about change of form {heterodont dentition). 

 The teeth of the premaxillaries and their 

 antagonists in the lower jaw are single- 

 rooted and usually have more or less a 

 chisel shape, hence they are called incisors 

 even when, as in insectivores, the crowns 

 are needle-like and there are two roots 

 (some insectivores, marsupials, lemurs) 

 (fig. 612). Behind the incisors (in the 

 maxillary bone in the upper jaw) is the 

 canine tooth (fig. 6or, c), which is single- 

 rooted and has usually a conical crown 

 (probably a modified premolar). Follow- 

 ing the canine come the molars, broad teeth 

 mostly with two roots and tubercular crowns. Only the anterior ones 

 appear in the milk dentition, while the others appear only in the perma- 

 nent dentition and are not replaced. On this developmental basis the 

 molars are divided into premolars (bicuspids of dentists), which appear 

 in both dentitions, and the true molars, which occur only in the last. 



As stated above, every species of mammal has a characteristic denti- 

 tion; its features may be expressed by a short formula. It is only neces- 

 sary to place the number of each of the four kinds of teeth mentioned in 

 their regular order, those of the upper jaw separated from those of the 

 lower by a horizontal line, to express this. Since the two sides of the body 



Fig. 601. — Permanent and 

 milk dentitions of the cat (from 

 Eoas). c, canines; p'~p'\ pre- 

 molars; vt', molar (tlie milk 

 dentition darker and each letter 

 preceded by d). 



