54 



Basic Structure of Vertebrates 



HSpgMp 



pm 



Fig. 53. Teeth of dog. (A) In profile. (B) Upper teeth seen from the cavity of 

 the mouth, (c) Canines; (i) incisors; (m) molars; (pm) premolars. (Courtesy, 

 Wiedersheim: "Grundriss der vergleichenden Anatomie der Wirbeltiere," Jena, 

 Gustav Fischer.) 



the lingual side of the old tooth. In reptiles, so far as is known, there 

 is indefinite replacement, but probably less frequently than in fishes, 

 especially when the teeth are large. In mammals one replacement is 

 the limit (Fig. 40). Most mammals develop a set of deciduous or 

 "milk-teeth" which, in early life, are replaced by the definitive or 

 permanent dentition — but, unfortunately, not always literally "per- 

 manent." If a tooth of the second set is lost, whether by accident or 

 old age, it is not replaced. Quite commonly the second dentition not 

 only replaces the "milk-teeth" but adds some cheek-teeth behind 

 those of the "milk" series. Teeth which replace "milk" cheek-teeth 

 are called "premolars." Cheek-teeth added behind the "milk" set 

 are called "molars" (Figs. 39, 40, 43). In some mammals there is little 

 or no replacement. A dentition developed in the young animal is re- 

 tained indefinitely. 



Vertebrates having indefinitely numerous replacements of teeth 

 are called "polyphyodont." Mammals in which deciduous teeth are 

 replaced by a second dentition are diphyodont, and those which 

 develop only one dentition are monophyodont. But the distinction 

 between the diphyodont and monophyodont conditions is not absolute. 

 In so-called "monophyodont" mammals, embryonic tooth-germs are 

 commonly found, in some cases on the lingual side and in others on 

 the labial side of the functional dentition. In embryos of diphyodont 

 mammals, occasional tooth-germs may be found either lingual to the 

 permanent dentition or labial to the "milk" dentition. The presence 



