330 DE. G. ELLIOT SMITH ON 



first makes its apjiearance in the kitten's brain. But in other Marsupials (e. g. Macroptis, 

 Thylacinus) the sulcus extends up^\'ar(;Is, and frequently crosses the ujDper border to 

 reach the dorsal surface of the hemisphere. 



In the larger Chiroptera we find a very extensive and fully-developed splenial sulcus 

 in a brain which lacks a genual fissure. Turner has figured it in Cynonycteris collaris 

 and Pteropus medius *. I have examined and found practically the same arrangement 

 in Pterojjus poliocejihalus. 



There is a general tendency, therefore, in most of the mammalian orders to the 

 formation of a definite splenial sulcus upon the mesial surface of the hemisjihere. We 

 find this tendency clearly expressed in the Edentata as well as in the Marsupialia, 

 Ungulata, Cetacea, Carnivora, and Chiroptera. It is therefore the expression of a very 

 general tendency among the Mammalia, and the fact that this fissure develops upon 

 parallel lines in the different families of Edentates must not be considered as an indica 

 tion of their mutual afiinity, but rather of their wider kinship with the general body of 

 mammals. In the Monotremata, or rather in Echidna, which alone of the two genera 

 possesses a convoluted brain, we find no definite splenial sulcus, and this absence seems 

 rather to accentuate the closer interrelationship of the other mammals, in which the sulcus 

 in question is never lacking in a convoluted brain. 



If we examine the brain in the large series of mammals which possess some representa- 

 tive of this sulcus, we shall find in many and very diverse orders an extensive and 

 regularly-arched sulcus extending forward in a " genual " manner such as we have 

 met in Orycterojms and Xenurus. We have already noted the existence of such a 

 splenial sulcus in Pteropus and other large Chiroptera; in many Cetacea we find a 

 similarly extensive and regularly arcuate splenial sulcus, as for instance Turner's figure 

 of BalcBHoptera f shows ; we find the same arrangement in many Ungulata, as for 

 instance in Eqmis, and among the Carnivora we find an admirable example of the same 

 arrangement in Proteles cristatus +. In the latter brain Krueg distinguishes the anterior 

 part of the large arcuate sulcus as genual, and the rest of the fissure as splenial. The 

 inconstancy and variability of the genual sulcus show^ that it is of very slight morpho- 

 logical importance, and in many cases it may be merely a mechanical product. The 

 splenial sulcus, howevei% is a much more definite and constant feature, and is obviously of 

 great morphological significance. But the most constant part of the splenial sulcus is 

 that portion which lies behind and below the splenium of the corj)vis callosum. In a 

 series of mammalian brains this part of the sulcus is absolutely constant in all hemispheres 

 which have any pallial sulci whatever ; it is also the deepest part of the sulcus ; it is, as 

 we have seen in the Cat, the first part of the sulcus to appear in ontogeny, and from 

 comparative studies we find that many brains (e. g. Phascolaretos) which possess no other 

 true pallial sulci have a small splenial sulcus in this situation. The posterior or ventral 

 part of the splenial sulcus is the constant element, but the anterior part is much more 



* Turner, op. cit., Journal of Anatomy and Physiology, vol. sxv. 



t Fig. 33. 



t W. H. Flower, " Anatomy of rrotdcs," Proc. Zoo), f-'oc. London. lS6i>, fig. 4. 



