106 DR. B. A. BENSLEY OX THE EA'OLUTIOX 



lead one to regard tlie condition in Myrmecobius as i^rimitive. That it is not so, however, 

 may be seen from a comparison of the Peramelidre, certain species of which present 

 canine modifications of the same kind and even of a more extreme degree. An 

 example is given in text-fig. 2 of tlie teeth of Chceropus castanotis. In all cases among 

 the Peramelidoe the modification is not only secondary hut also purely local. 



Fremolars. — Apart from a marked tendency to develop accessory cusps, which is 

 doubtless due to their separation by diastemata, these teeth present the same characters 

 as those of the Dasyurina?. As already mentioned above, they decrease in size from 

 before backwards. The exact explanation of tliis condition is doubtful ; the small size 

 of the posterior tooth may be due simply to the circumstance that it forms an inter- 

 mediate member between the larger anterior teeth and the greatly reduced molars, or 

 that it has been formerly reduced as in the normal Dasyurinae. As regards the presence 

 of accessory cusps, it may be observed that exactly the same tendency is found in the 

 Peramelidae tinder similar conditions of increased space in the premolar region. 



Milk premolars. — In describing the dentition of the TDasyurinae no reference has been 

 made to the modifications of the deciduous teeth for the reason that their characters do 

 not joertoin to the category of secondary adaptations here considered, but to the primary 

 differentiation of the Marsupials as a group. Some reference to them is, however, 

 necessary in the case of ^Ji/rniecobins, on account of their possible connection Avith the 



origin of the increased molar formula. 



mp. ap. 



Fi! 



mp. ap. 



^^^l^^A 



Doubtful case of dental replacement in Mi/rmecobius fasciatus. 



a. Internal view entire left ramus of jaw of young sjiecimen (nat. length of jaw=:23 mm.) ; b. External view of part 

 of right ramus of same specimen ; c. Right ramus of older specimen ; d. External view of right posterior 

 premolar and anterior molars of adult animal. Abbreviations : (qK, mji., jip., anterior, median, and posterior 

 premolars ; d., doubtfully deciduous premolar ; m. 1, first true molar. All figures drawn to same ampliiication. 



The milk-premolars have been recognized by Woodward (i 896) in the young, but their 

 subsequent fate has not been elucidated. According to the view of Thomas (1B87) and 

 Leche (1891) they are replaced by the posterior premolars in the ordinary way, while in 

 Winge's (1882) opinion they remain in place, the posterior premolars being unable to 

 dislodge them on account of the elongation of the jaw. 



The accompanying text-fig. 3 represents the anterior molariform teeth of the lower 

 jaw in three specimens, all of which have been drawn to the same amplification. Of 

 these a and b represent the teeth of a young specimen, in which the posterior premolars 

 are not yet formed. In a the first member of the molariform series is comparatively 



