104 ILLINOIS STATE ACADEMY OF SCIENCE 



posterior or temporal end of the hippocampus. In this 

 brain the temporal end of the hippocampus, which marks 

 the morphological anterior end of the temporal lobe in 

 higher mammals, lies very near the posterior pole of the 

 hemisphere, a condition recalling that found in the rep- 

 tiles. There is only a very slight forward curvature of 

 the hippocampal formation in this region. The hippo- 

 campus is, as it were, caught in the very act of "turn- 

 ing the corner". The opossum hippocampus has already 

 done so very neatly, by means of the formation of a for- 

 wardly directed pouch of cells. 



Turning now from a study of the general anatomy of 

 this brain we may consider the second phase of interest 

 connected with it, which concerns the light which it might 

 shed upon the di-polyprotodont controversy, with its 

 important bearings on the geographical and chronologi- 

 cal distribution of marsupials. The most exciting single 

 feature of the brain of Coenolestes is that one which con- 

 stitutes the evidence on this point — the only evidence, 

 apparently. Writing in 1902, Prof. Elliot Smith stated 

 that upon examination of all marsupial brains (except 

 Coenolestes) he found that all diprotodont brains with- 

 out exception possessed an anatomical peculiarity which 

 was not to be found in any polyprotodont brain or in 

 any other mammalian brain, and which therefore consti- 

 tuted a true diagnostic character of diprotodont brains. 

 This structure he named the 'aberrant bundle". It is 

 the upper part (Fig. 5, f. ab.) of the anterior or ventral 

 commissure which splits away from the lower part to 

 gain the corona radiata by way of the internal capsule 

 (cap. i) instead of by the usual route in the external cap- 

 sule (cap. e). It appears in every diprotodont brain re- 

 gardless of size, from the giant kangaroo, five feet long 

 from snout to root of tail and weighing 200 pounds, to the 

 pygmy flying phalanger, two and a half inches long ex- 

 clusive of the tail. It is absent in every polyprotodont, 

 American or Australian, including the largest, the Tas- 

 manian wolf, which it is said a number of dogs will hesi- 

 tate to attack. I have not been able to detect it in Coenol- 

 estes. If it be accepted as a criterion of diprotodont 

 brains, Coenolestes should be classified with the poly- 



