ISG DE. B. A. BE^'SLEY OX THE EVOLUTION 



mentioued as prototypal for the Australian series, as, for example, the tritul)ercular type 

 of upper molar ; (e) those which iudicate a similar potential of evolution, such as the 

 reduction of the posterior premolars and the development of syndactylous modifications 

 in the pes. It will be observed that all the characters of the dentition and foot- 

 structure described as prototypal, and distributed over three families in the Australian 

 series, may occur together in the Didelidiyidie in a single species. 



Deiitition. 



Ilolar Palienis. — 'J'he upper molars of the smaller Dasyurinse have already been 

 mentioned as completely prototypal, except in the arrangement of the external styles. 

 We may now notice the fact that their patterns are practically identical with those of 

 Feratherium and the existing Didelphyidae {cf. PI. 5. figs. 1 a-h, 2 a-b, 26, of Peratherium, 

 Smuitho2)sis leucopus, and Metachlrus opossum). The only important exceptions to 

 this general statement are as follows : — In the first jilace, the molars of the more advanced 

 Didelphyidae (tig. 2G), while diifering from those of the Australian forms (advanced 

 Dasyurinse and Peramelida)) in tending to retain the original insectivorous character, 

 show less antero-posterior compression than those of the smaller forms, and also 

 Peratlierlum and the smaller Dasyurinae. Secondly, one member of the family, Caluromys, 

 shows indications of an omnivorous development, wdiich, if continued further, w^oidd 

 parallel that of the Phalangerina3. Thirdly, there are minor variations in the 

 arrangement of the external styles ; all of them, however, both in tbe Didelphyidae and 

 the Australian forms, are easily traceable to the Pera^//,ermm-type. 



Comparing the second molar of Sirunthopsis leiicopus here figured with that of 

 Peratherlmn, it is seen that in the former styles a and h are fused together. Intermediate 

 styles are not represented. Style c is greatly enlarged. In other members of the 

 Dasyurinte style c^ is occasionally developed. In the British Museum Collection it; is 

 present in m. 2 of eight specimens of Phasrogale Jlatipes^ in m. 1 and m. 2 of one 

 specimen of P. Sivainsoni, in m. 1 of three (of five) specimens oi P . penicillata, and in 

 m. 2 and m. 3 of two. One specimen of P. minutissiina shows both style Cj and b., in 

 m. 1. Style c^ is generally absent in the smaller forms. Even in Peratherium it is so 

 small as to be scarcely recognizable. .It is altogether absent in all of the British Museum 

 specimens of Sminlliopms. It is present inA«.2in one specimen P. Swainsoni; it is 

 absent in all of eleven specimens of P. flavipes. In Phascogale j^^nicilluta, the type 

 figured by Winge in connection with this element, it is present in m. 1 and m. 2 in three 

 of five specimens. It is absent in m.S of three specimens, and in each of the two others 

 it is represented by two minute tubercles. It is present as a trace in m. 1 and m. 2 of 

 one specimen of P. dorsalis and one of P. Thorheckiaua, and is barely indicated in most 

 specimens of JJasyimis [cf. PI. 5. fig. 3). 



The arrangement of the external styles in the existing Didelphyidae has already been 

 referred to. It needs only to be added that not only are the variations traceable to 

 a Perat/ieriuni-ty])e, but aho that theie is less departure from that type than is seen in 

 the Dasyurinae. The same tendency towards the fusion of styles a and b is noticealde 

 {rf. PI. 5. figs. 28, 29), but is not so apparent as in the Dasyurinae. The couditiun is 



