181, DE. B. A. BENSLEY ON THE EVOLUTION 



developed condition of style Co in Marmosa mnrina, M. chierea, M. rapposa, and in some 

 specimens of Galuromys in reality represents a general levelling down of the whole stylar 

 series prior to its obliteration. In some specimens of Caluromys, as in that here figured, 

 the clngular ridge is seen to be extremely small and thin, and to bear, instead of definite 

 styles, a number of minute crenulations. All of the molar characters of this genus 

 are indicative of an incipient omnivorous development, pointing towards that seen among 

 the Australian Marsupials in the Phalangerinse, where, even in the most primitive forms 

 {of. PI. 5. fig. 13, Distoechnrus), the external styles have already disappeared. 



In the writer's opinion the relationships of the Didelphyidae are as indicated in the 

 following modified plan : — 



Chironectes. Dir]eli)hyg. 



Caluromys. 



Dromiciops. 



'Peramys. 

 Marmosa. 



This represents an incipient radiation, since the diverging lines are directly traceable 

 to Marmosa. It is an interesting fact that the representatives of these lines are still 

 scarcelv separable systematically from that genus. As in the Australian families, 

 increase in size of the body appears to have been an essential feature of the evolution. 

 With the exception of Dromiciops, the two genera Marmosa and Peramys include the 

 smallest and most primitive forms of the family. Peramys bears much the same relation 

 to Marmosa as Sminthop)sis does to Phascogale, being in general more primitive in 

 dentition, but specialized in a terrestrial direction in foot-structure. The most important 

 dental character demonstrating the prototypal position of Peramys relates to the posterior 

 premolars, which are, as a rule, larger than the median teeth. In Peratherium they are 

 apparently always larger, but in Marmosa, as well as in the more specialized forms, they 

 show definite signs of reduction. The foot-characters in which Peramys is specialized 

 relate to the redviction of the terminal phalanx of the hallux, reduction of the striated 

 plantar pads, and shortening of the fifth digit, all of which modifications are connected 

 with a secondarily terrestrial habit. Peramys is also more specialized than Marmosa 

 in the shortened, hairy, and, apparently, non-prehensile character of tlie tail. It is 

 interesting to note that in the Didelphyida? the tail ranks with the teeth and feet as an 

 oro-an of adaptive change. As indicated in Winge's plan, Marmosa represents the 

 parent form of all the remaining genera. The existing species have, however, 

 undergone a specialization of the posterior premolars, a reduction similar to that found 

 on a larger scale in Phascogale. Some of the species have further undergone a special 

 development of the molars, canines, and anterior premolars pointing towards the genus 



