188 TEN years' progress in vertebrate paleontology 



Maesupialia 



The memoir by Bensley, "On the Evolution of the Australian Marsu- 

 pialia" (1903) is a notable attempt to interpret the Marsupials from the 

 viewpoint of adaptation and phylogeny. Dollo had shown that the 

 peculiar s^oidactylous modification of the third and fourth digits of the 

 pes in x4ustralian Diprotodonts is very probably a reminiscence of former 

 arboreal habits. Bensley extended Dollo's observation and showed that 

 even among the still existing Marsupials there are many gradations be- 

 tween the normal five-toed pes of the Didelphids and the highly special- 

 ized pes of the kangaroos. He also described the supposed changes in 

 the dentition, starting with the small insectivorous didelphids with so- 

 called tritubercular dentition, passing through various intermediate 

 types in the several lines and culminating respectively in the specialized 

 carnivorous t3^pes with sectorial molars, in the specialized herbivorous 

 types with bilophodont molars, in the root-eating wombats with rodent- 

 like dentition, and so forth. 



The hj^othesis according to which these results were worked out was 

 that the tritubercular dentition had been the starting point in the Aus- 

 tralian Marsupials as in the Placentals, and that the steps by which 

 various specializations had been attained in the Placentals had also been 

 paralleled more or less in the Marsupials. 



The scant paleontological data are by no means unfavorable to Bens- 

 ley^s conclusion.^ The didelphoid type, which he regards as the ances- 

 tral form, had a wide northern distribution in the Oligocene, Eocene, 

 and very likely also in the Cretaceous. The Tasmanian Tertiary genus 

 Wynyardia of Spencer, known from a well preserved skull, which unfor- 

 tunately lacks the teeth, appears to connect the diprotodont with the 

 pol}'protodont type.* 



The close relationship of diprotodonts and polyprotodonts is virtually 

 proven by the uniform and highly characteristic arrangement of the 

 cranial foramina,^ by the peculiar form of the orbitosphenoid,® and by 

 many other important agreements, both in structure and in development,'^ 

 so that it would not be surprising if Bensley should be proven to be right 

 in inferring that the Australian marsupial fauna is of Tertiary deriva- 

 tion and evolution. 



Passing to the consideration of works on fossil Marsupials, we notice 



3 Gregory, op. cit., pp. 205-206. 

 * Ibid., pp. 214-215. 

 5 Ibid., pp. 222-225. 

 «lbld., p. 245. 

 Ubid., p. 221. 



