530 



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 

 (i., c.,p., ra.), 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: 



i. ?* c . ri , p. , m. 3 ' 3 = 44, 



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

 are always the same 



.3143 



i. , c. -, p. m. = 44 

 o 1 4 o 



Echidna has no teeth at any stage. In Ornithorhynchus teeth 



are present in the young, 



but are early absorbed, 



and the function of teeth 



is performed in the adult 



by broad horny plates, 



two on the upper and 



two on the lower jaw. 

 The Marsupials have 



the milk dentition in 



a degenerate condition. 



Germs of milk teeth are 



developed, but with the exception of one, the last pre-molar, and 



in some cases of canine and incisors, these remain in an im- 

 perfect state of development, 

 though they persist, as func- 

 tionless vestiges, to a compara- 

 tively late stage. 



In the adult dentition of the 

 Marsupials the number of in- 

 cisors in the upper and lower 

 jaws is always dissimilar ex- 

 cept in Phascolomys. With re- 

 gard to the arrangement of 

 these teeth, the order falls into 

 two series termed respectively 

 the diprotodont and the poly- 

 protodont. In the former (Figs. 

 1118-1119) the two anterior in- 

 cisors are large and prominent, 

 the rest of the incisors and 

 the canines being smaller or 

 absent. On the other hand, in 

 the polyprotodont forms, which 



FIG. 1117. Teeth of Bandicoot (Perameles). 

 (After Owen.) 



FIG. ni8.-Front view of skuii of Koala 



(Phascolarctos cinereus), illustrating dipro- 

 todont and herbivorous dentition. (After 



