IV. VERTEBRATA: MAMMALIA. 



625 



is constant for the species, usually for the genus, and often for 

 the family. As man normally has thirty-two teeth, so the dog 

 has forty-two, the anthropoid apes thirty-two, the platyrhine apes 

 thirty-six, etc. (2) The teeth are firmer. The body of dentine 

 is divided, by a slight constriction, into a crown covered with 

 enamel, and a root enveloped in cement (bony tissue). The roots 

 are placed in separate sockets (alveoli) in the jaws, and in those 

 cases where continuous growth is necessary the pulp persists and 

 the teeth, as in the incisors of rodents and the tusks of elephants 

 and pigs, grow indefinitely. (3) In consequence of their greater 

 firmness the teeth are not used up so fast and do not require rapid 

 replacement. There occurs only one change, in which the denti- 

 tion 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 

 (monophyodont 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 of 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 

 dentition following the permanent one is outlined in many placen- 

 talia, 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 in- 

 sectivores, the crowns are needle-like 

 (fig. 661). Behind the incisors (in the 

 maxillary bone in the" upper jaw) is 

 the canine tooth (fig. 650, c), which is 

 single-rooted and has usually a conical 

 crown (probably a modified premolar). 

 Following the canine come the mo- 

 lars, broad teeth mostly with two roots FIG. 650. Permanent and milk 

 , J _ _ ,. dentitions of the cat. (From 



ana tubercular crowns. Only the an- Boas.) c, canines; p a -p 4 , pre- 



, ., .,, _ molars; m', molar (the milk den- 



terior ones appear in the milk den- tition darker and each letter 



tition, while the others appear only in 



the permanent dentition and are not replaced. 



preceded by d). 



On this develop- 



