118 DR. B. A. BEXSLEY ON THE EVOLUTION 



full number of incisors was present in the upper jaw, whilst in only a comparatively 

 few specimens an additional premolar was present in the lower jaw, always small and 

 imshed out to the side of the jaw in front of the first molar." The formula given by 

 Spencer differs only from that determined by Gadow from Stirling's specimens in the 

 addition of one upper incisor and one lower premolar. 



Gadow has expressed the opinion that the affinities of Notoryctes are with the 

 Dasyuridje, and the subsequent discovery of a fuller incisor formula of 5 appears at 

 first sight to support this view. It must be borne in mind, however, that the upper 

 incisor formula of 4 only represents one limit, so far as known at present, of the varia- 

 bility whicli is normal for this genus; it cannot be regarded as indicating the presumably 

 stable formula of the ancestral form from which Notorydes has been derived. Further- 

 more, no greater importance can be attached to the occurrence of a fourth upper incisor 

 in one of twenty-nine specimens than, for example, to the occasional presence of a 

 fourth lower incisor in Ifyrmeeohius or a fifth in Didelphys, even though the latter be 

 regarded as reversional, a proposition which, as Bateson (1894) has shown, is rather 

 doubtful. 



9 



With reference to the premolar formula, Gadow considered the number ^ determined 

 by him to represent the anterior and median premolars of other Marsupials, the posterior 

 teeth having disappeared as in the Dasyuridse. This, again, cannot be regarded as 

 indicating a special affinity with the latter family, since the same process of reduction 

 of the posterior premolars is found in other families. In the Phalangeridse the genus 

 Acrohates shows a reduced condition of the posterior premolars, and in Dlstcechui-m 

 (of. PI. 5. fig. 39) the upper teeth are much reduced and the lower are absent, as in the 

 more advanced members of the Dasyuridae. The same tendency towards reduction is 

 seen in the Didelphyidae, and in two species, Thylacomys lagotis and Clioeropus castanotis, 

 of the Peramelidse. 



The dental reduction of Notorydes appears to be confined to the antemolar teeth, the 

 molars always presenting the formula 4 characteristic of all of the remaining Marsupials 

 with the exception of Mynnecohlus, Acrohates, Distoechurus, and two species of 

 Dromicia. 'Ihe cause of the stability of the molar formula is to be sought in the 

 o-reater functional importance of these teeth. Like the antemolar teeth they have been 

 atfected by the shortening of the jaws, but only to the extent of an antero-posterior 

 compression. 



Derivation of the Molar Patterns. — The characters of the upper teeth are represented 

 in PI. 5. fig. 12, and those of the lower in PI. 6. fig. 17. 



The appearance of the molar crowns has been figured by Stirling (1891) in connection 

 with his original description. Tomes, however, remarks that, "in Dr. Stirling's figures 

 of the grinding-surfaces of the molar teeth, it is shown that the middles are worn into 

 concavities, and that the retention of the cuspidate form is not due to the retention of 

 sharp enamelled cusps, but tliat it is due to the upstanding of the edges." Wliile this is 

 probably true of the lower molars figured by Stirling, the prominences shown in his 

 illustrations of both upper and lower teeth will be found to correspond with those here 



