BY G. ELLIOT SMITH. 643 



In Perameles the sagittal section of the commissure is thin in 

 front, where it is composed of scattered bundles coming from the 

 anterior part of the hippocampus. But as it is traced back it 

 becomes thicker and more compact from the crowding together of 

 fibres from the middle portion of the hippocampus. Posteriorly 

 the commissure takes a sudden bend downwards and forwards, 

 and at the bend a large number of fibres are aggregated to form 

 a structure which resembles the spleniuin corporis callosi of 

 placental mammals, and was actually believed by Flower to be the 

 splenium. Since it is produced in the same manner as the 

 splenium, it may not be inappropriate to call it the "splenium of 

 the hippocampal commissure." From this splenium the ventral 

 limb of the commissure extends downwards and forwards like the 

 psalterium of the placental mammal, and as in the latter it is 

 formed mainly by the fimbria. In the higher Marsupials the 

 angle formed at the splenium by the two layers of the commissure 

 becomes more acute, and in the highest Metatherians like the 

 Kangaroo the two layers form two parallel bands united behind 

 at the splenium. After this description of the arrangement in 

 PerawpJes, the appearance of the commissure met with in Platypus 

 will 1)6 readily understood. As the descending horn of the 

 ventricle is rudimentary there is a very small descending limb of 

 the hippocampus. There is consequently no ventral layer of the 

 commissure, but only a slight hook-like " splenium " at the posterior 

 extremity of the commissure. In the Eutheria (fig. 3) where that 

 portion of the hippocampus lying above the velum interpositum 

 does not develop, there is consequently no dorsal layer of the 

 hippocampal commissure; but, corresponding to the ventral part 

 of the hippocampus, which alone persists, the ventral layer of the 

 hijjpocampal commissure forms the psalterium, as we usually 

 understand that term — the place of the dorsal layer of the com- 

 inissnre being occupied by the corpus callosum. The corpus 

 callosum of higher mammals is continuous with the psalteriimi at 

 the callosal splenium, and the exact point of union of the two is 

 not usually evident. Although in the highest mammals the 

 splenium is formed wholly of callosal fibres, yet in some of the 



