MACEOPODIB^. 



be sought for if still in the bone, although at still younger stages its 

 position is generally rather more above p.' than m-p/, owing to the 

 fact that allowance has to be made for the movement forwards of 



Fig. 2. 



Macropus rufieollis. — A and B as in fig. 1. 



the tooth-row, by which m.p/ will be brought opposite the suc- 

 ceeding p."* just at the time that the latter tooth is ready to emerge 

 from the bone. 



The time of the appearance of p.* varies in the different members 

 of the family ; but for the most part the tooth is cut shortly after 

 m.^, and always before m.* is in place. 



The movement forward of the whole row of teeth is also a point 

 which should be specially noticed, in order that the common error 

 of taking as a systematic character the relative position of indi- 

 vidual teeth to the skull maybe avoided — such relative position, 

 although of importance in other groups, being entirely valueless 

 in the more typical Macropodidoe. It should, however, be observed 

 that the general position of the tooth-row as a whole and the length 

 of the diastema are both fairly constant throughout the life of the 

 animal, partly owing to the fact that at the tooth-change two teeth 

 (p.' and m.p.'') fall out, and only one (p.') succeeds, and partly to the 

 counteracting effect of the longitudinal growth of the jaw-bone. 

 Moreover, in certain of the larger species, in which the pace at which 

 the teeth move forwards is far greater than in the smaller ones, p.* 

 itself, and either one or two of the anterior molars, may also fall 

 out as the rest move forwards — a process by which the distance 

 from the anterior cheek-teeth to the front of the mouth is always 

 kept about the same. 



Another point to which attention should be directed is that the 

 molars, especially of the larger species, emerge from the bone with 

 their hollows nearly entirely filled up with cement, external to and 

 quite distinct from the enamel of the tooth. The result of this is 

 that some specimens have the cusps and ridges of their molars 



