166 DR. B. A. BENSLEY ON THE EVOLUTION 



habits of the animal are terrestrial. Its foot-characters appear to repi'esent an inter- 

 mediate stage between the condition in Phascogale and that in the most primitive form 

 [S. leucojjus) of S7uodho2Jsis, with, however, a substitution of a plantij^rade for the 

 saitatorial modification which distinguishes the latter genus. 



Passing from Fhascogale to Sminthopsis and Antecliinomys, we meet with a com- 

 paratively well-graded series of minute forms which show a terrestrial and saitatorial 

 evolution. The most primitive co]idition is found in Sminthoiisis leucopus (PI. 7. fig. 3), 

 the foot-structure of which approximates very closely to the type presented by 

 JPhascogale flavipes. The foot is relatively longer than in P. flavipes, the proportion 

 being 1 : 5'5. The liallux is slightly more reduced and its terminal pad scarcely 

 swollen. The plantar surface is not vrholly naked as before, the marginal hairs of both 

 sides of the foot showing a tendency to encroach on its posterior portion. The heel 

 is almost completely hairy. The hallucal pad is scarcely distinguishable from the 

 surrounding granular surface ; its striated area is minute and oval. The digital pads 

 are as well marked as in 'P. flavipes, but are much more rounded and less distinctly 

 separated. The striated surface does not extend over the whole pad, but is confined to a 

 small area at the apex, the granulated surface having encroached on the base. The 

 marginal pad is practically absent, its position being only indicated by certain larger 

 granules. 



o 



Sminthopsw larapinta while agreeing with S. leucopus in the proportions of the foot 

 (1 : 5'1) presents a more advanced stage in respect to the characters of the pads. The 

 hallucal pad is practically absent, being only represented by a row of three large smooth 

 granules. The digital pads are closely set together and their bases are fused. Tlie 

 basal granules of each have now encroached to such an extent that the formerly well- 

 developed and striated surfaces have become much reduced. The marginal pad is 

 absent; its former presence is indicated by one or two larger granules. 



Sminthopsis murina shows a variation or side development of the condition seen in 

 S. leucopus. The proportion of breadth to length is 1 : 5. There is a small elevation at 

 the base of the hallux, with a larger granule anteriorly. The digital pads are well 

 developed and separate, except at their bases; but their surfaces are completely 

 granular. There is in each case a larger granule representing a formerly striated area. 

 The marginal pad is al)sent. 



In Sminthopsis crassicaudata (PI. 7. fig. 4) we find a more advanced stage than that 

 in S. leucopus and S. larapinta. The ratio of breadth to length is 1 : 7"5. Except for a 

 sUghtly enlarged granule at the base of the hallux, the hallucal pad is unrepresented. 

 The digital pads are still more completely fused together basally. Their surfaces are 

 entirely granular, but the apex of each is occupied by a large differentiated granule, 

 which obviously represents the remains of a formerly striated surface, as in ^S". murina. 

 The marginal pad is wholly absent. 



