ZOOLOGY 



SECT. 



The number of the various sets of teeth in the jaws is con- 

 veniently expressed by a dental formula, in which the kind of tooth 

 (incisor, canine, pre-molar, molar) is indicated by the initial letter 

 (, c. y p., m.\ and the whole formula has the arrangement of four 

 vulgar fractions, in each of which the numerator indicates the 

 teeth of the upper, the denominator those of the lower jaw. Thus : 



. 3'3 1-1, 4-4 3-3 



^F3' c Tl^^ m >3 = 44 - 



or, in a simpler form, since the teeth of the right and left sides 

 are always the same 



FIG. 1186. Teeth of Bandicoot (Perameles). 

 (After Owen.) 



Echidna has no teeth at any stage. In Ornithorhynchus teeth 



are present in the young 

 and are functional for 

 a time, but they are 

 thrown off when the 

 animal is about a year 

 old : vestiges of an 

 earlier dentition have 

 been detected, and the 

 function of teeth is per- 

 formed in the adult by 

 broad horny plates, one 

 on each upper and one on each lower jaw. 



The Marsupials have the milk-dentition in a degenerate condition. 

 Germs of milk-teeth are de- 

 veloped, but with the exception 

 of one the last pre-molar 

 and in some cases of canine and 

 incisors, these remain in an 

 imperfect state of development, 

 though they persist, as function- 

 less vestiges, to a comparatively 

 late stage. . 



In the adult dentition of the 

 Marsupials the number of in- 

 cisors in the upper and lower 

 jaws is always dissimilar except 

 in Phascolomys. With regard to 

 the arrangement of these teeth, 

 the order falls into two series, 

 termed respectively the dipro- 

 todont and the polyprotodont. Fjo 1187> _ Front view of skull of K oaia 



In the former (FlgS. 1 187-1188) (Phascolarctos cinereus), illustrating dipro- 



v . e . . todont ar.d herbivorous dentition. (After 



the two anterior incisors are Flower.) 



