OF THE AUSTRALIAN MAESUPIALIA. 



95 



111 the first molar {rf. PI. 6. figs. 7 & 8, Basyurus Geoffroiji and Sarcophiliis 

 urslnus) there is a further reduction and final ohliteration of the paraconid and 

 metaeonid, the tooth assuming a premolar condition. In the fourth molar the talonid 

 becomes reduced to a vestige. 



That the above-mentioned modifications, with the partial exception of those of the 

 first lower molar, represent the successive phases of an insectivorous-cai-nivorous 

 evolution, can be readily shown by reference to their functions. The minute teeth of 

 Sminthopsfs leucoj)us serve three distinct purposes, adapting them for the comminution 

 of insect-prey. A piercing action is performed by all of the cusps of the relatively 

 broad crowns, a shearing one by the Avorking of the metacone-spur of the upper molars 

 against the paraconid and protoconid of the lower, and finally a crushing action is 

 performed by the working of the protocone of the upper molars into the talonid of the 

 lower. In the extreme members of the scries both the piercing and crushing actions 

 are abandoned, and the shearing action perfected. The lapse of the crushing function 

 is indicated by the reduction of the protocone and talonid, while the lapse of the 

 piercing action and the perfection of the shearing one is proclaimed by the lateral com- 

 pression of the teeth and the conversion of the originally conical cusps into trenchant 

 blades. The reduction of the metaeonid is also attributable to the lapse of its piercing 

 function, there being in the advanced stages no cusps against which it might shear. 

 The rotation of the metacone-spur is no less a character of carnivorous adaptation than 

 the trenchant modifications of the cusps : by meaus of it the teeth are made to shear 

 together in a single longitudinal line instead of individually in parallel transverse lines, 

 as is the case wdth Sminthopsis and its allies. 



The changes which take place in the first lower molar, and result in the conversion of 

 a typical tuberculo-sectorial tooth into one of premolariforra character, are associated 

 with the reduction of the posterior premolars shortly to be described, and are therefore 

 only indirectly connected with the carnivorous evolution. In the smaller species of 

 Pluiscogale, and in Sminthopsis and Antechinomys, where the upper posterior premolars 

 are well developed, or at most only slightly reduced, the latter teeth shear against the 

 protoconid and paraconid of the first lower molars in much the same manner that the 

 metacone-spur does in the succeeding teeth. With the reduction of tlie posterior 

 premolars in the larger forms this function is obliterated, and the protoconid loses its 

 shearing character, while the paraconid disappears. The metaeonid becomes reduced 

 for the same reason as in the remaining teeth. In Dasi/iiriis and Sarcophiliis, where the 

 posterior premolars are entirely absent, the first lower molars bite loosely against the 

 median upper ones. 



Incisors. — These teeth present a carnivorous evolution almost as markcHl as that just 

 described for the molars. The change alfects chiefly the relations of the median upper 

 teeth and the length of the tooth-rows. Throughout the species of Phascoyale the 

 median upper incisors are sharply differentiated from the lateral teeth, being elongated, 

 procumbent, and subcaniniform. They are separated at their bases and aj)proximated 

 at their tips. The lateral incisors arc short and of the usual spatulate type, except that 



