:.G2 



ZOOLOGY 



SECT. 



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

 veniently' exjiressed by a dental forniida, in which the kind of tooth 

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

 (/., c, It., 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 



'■ 8-3' ' 1-1 ^'4-4' ■ 3-3 

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

 are always the same — 



.314 3 



= 44. 



3' 'Vi' ': 

 Echidna has no teeth at any stage. 



:^fg^; 



-Tcetli of Bandicoot (I'en 

 (After Oweii.) 



„ela). 



:44. 



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 jjlates, one 



on each ujDper and one on each lower jaw. 



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



Germs of milk-teeth are de- 



veloi^ed, but with the exception 



of one — the last pi-e-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 



Marsujiials the number of in- 

 cisors in the upj)er and lower 



jaws is always dissimilar except 



in I'lioscolomys. With regard to 



the arrangement of these teeth, 



the order falls into two series, 



termed respectively the dipro- 



todont and the volimrotodont. „ „„, _ , . , , ,i „f Tr„,ia. 



J- -V, ,_ TToo\ ^^"- llST.^Front view o£ skull of Koala 



Jn the I0rmer(r IgS. 1187— lloo) (nnamlarctm cinemin), mustrating dipi-0- 



, , , i_ ■ • ■ todoHt and herljivorous dentition. (After 



the two anterior incisors are Flower.) 



