310 DE- G. ELLIOT SMITH ON 



In reviewing the features presented by the cerebral commissures in the Edentata, we 

 find that, while each family presents a more or less distinct type of dorsal commissure, 

 all of the commissvires conform to the type prevailing in the Eutberia generally, and an 

 analogue for almost any Edentate type may be found among the Eutberia. 



There is no feature in the arrangement of the commissures which indicates a transition 

 stage between the primitive Saiiropsidan -like type which prevails in the Prototheria and 

 the Eutberian brain with a well-formed coi'pus callosum. It may be that the gradual 

 dwindling of the corpus callosum which is obviously going on in the Armadillos may 

 indicate the mode by which the Marsupials might have lost their corpus callosum, when 

 their dorsal commissure assumed a resemblance to that of the Monotremes in being purely 

 hippocampal. Eor the possibility of the Marsupials having once had a corpus callosum 

 and subsequently lost it is forced upon our consideration by the dwindling of the 

 anterior and dorsal parts of the hippocampal arc — changes which we associate causally 

 in the Eutberia with the development of a corpus callosum. But the discussion of the 

 question whether the ancestors of the Marsupialia originally had a corpus callosum 

 must be postponed for a future memoir. 



There are certain general questions relating to the cerebral commissiu-es which may be 

 discussed to better })urpose after we have considered the cerebral cortex. 



The Hippocampal Formation. 



We have seen elsewhere * that in the Monotremata and Marsupialia the hippocampus 

 retains a peculiarly simple arrangement with relation to the margin of the hemisphere, 

 which it has inherited from the ancestors of the Mammalia. 



In approaching the study of this region of the brain in the Edentata, I have deemed 

 it important to enter with some detail into the exact arrangement of the hippocampus, in 

 the hope that some indication of the affinities of the Order might be obtained. 



Extending downward and backward from the postero-iuferior aspect of the psalterium, 

 we find upon the mesial surface of the hemisphere (fig. 4, p. 291) two or more peculiar 

 arcuate bands, which represent all that can be seen upon the surface of the pecviliar hipjjo- 

 campal formation — the homologue of the hippocampus major of human anatomy. If, 

 before beginning the study of these ])eculiar surface areas of the hippocampal formation, 

 we examine this structure from within by opening up the cavity (lateral ventricle) of 

 the hemisphere, we shall gain a much clearer conception of the region than would be 

 the case otherwise. 



If we dissect away the lateral wall of the hemisphere of Ori/cterojjus so as to expose 

 the mesial wall of the lateral ventricle, we find a large crescentic white mass bulging in 

 the posterior part of the ventricle, looking not unlike the pupa of a silkworm lying in its 

 cocoon. This large curved swelling is the hippocampus. Its concave anterior border 

 is fringed by a prominent ridge of compactly arranged fibres — the fimbria. The lower 

 extremity of the swollen mass is pointed, and the upper extremity, which extends slightly 



* Cf. this Vol. p. 47. 



