348 DR- G. ELLIOT SMITH ON 



When we consider the close resemblance in shape between the brains of the Sloths and 

 the gigantic extinct Ground-Sloths, and in view of the fact that increase in bodily 

 dimensions in any group of mammals usually implies a more extensive pallium, we 

 mio'ht expect to fiiid in the brains of Megatherium, Mylodon, and Scelidothermm some 

 signs of a richer pattern of sulci and perhaps a definite medial sagittal sulcus. So far as 

 can be judged from the figures which Gervais gives of the cranial casts of these extinct 

 monsters, the brain of Mylodon appears to have been richly convoluted after the analogy 

 of the brain of the rhinoceros. While we can readily recognize the sagittal sulcus, we 

 cannot be sure of the presence of a definite mesial sagittal sulcus after the manner 

 of Cholaepus. In Scelidotherinm, whose brain is just like a very much enlarged brain of 

 Bradypus, there is a typical sagittal sulcus, but we can recognize no mesial sulcus. 

 The same remarks might ajiply to the brain of Megatlierium ; but the observer cannot 

 fail to be struck with the apparent simplicity of these two brains. In the brains of such 

 large animals, when we consider the well-developed sulcal pattern in the minute 

 Bradyjnis, we might have expected an extremely rich and complex pattern of sulci. 

 There appears to be some indication of this in the cranial cast of Mylodon. 



On the lateral aspect of the sagittal sulcus there is, in the brain of BiHidypus, a strongly- 

 arched sulcus whose features are very constant in all the brains which I have examined, 

 and of "nbich there are records as well in the cranial casts. It is unquestionably the 

 liomologue of the suprasylvian sulcus of tlie Carnivores, to which we have referred as S 

 in the other Edentate brains. It is interesting to note that it is strongly arcuate, like the 

 corresponding sulcus in the Carnivores and its rudiments in the Ant-eaters ; for it 

 therefore contrasts in a marked manner with the corresponding sulcus in the lowlier 

 Unffulates and the Aard-vark, in which it is horizontal. 



In both of the brains of Bradypus which I have examined, a deep furrow springs 

 from the angle of junction of the anterior and posterior rkinal fissures and extends 

 obliquely upAvard, with a slight inclination backward toward the central point of the 

 concavity of the sulcus S. This furrow we may for the present distinguish as the sulcus 

 /I. If we were considering the brain of a Carnivore or an Ungulate, there should be no 

 hesitation in calling the sulcus n the fissure of Sylvius (as that term is generally applied 

 outside the Primates), but there are reasons for some hesitancy in adopting this title 

 in Bradypus, because no other Edentate whatever has a Sylvian fissure. 



There is a considerable resemblance between the brains of Bradypus tridactylns and 

 the small Carnivore Genetta t'lgr'ma, of which Mivart has given some useful illustrations *. 

 The cranial aspect of the pallium of Genetta, the brain of which is not unlike that of 

 Bradypms in shape, is impressed by sasittal, suprasylvian, and supraorbital sulci, which 

 in simplicity are comparable with those of the Three-toed Sloth. The shape and 

 appearance of the suprasylvian sulcus and its relation to the simple oblique Sylvian 

 fissiire are exactly comparable to the arcuate sulcus S and its relation to the enigmatical 

 sukais n in Bradyprns. A comparison of my fig. 11 with Mivart's fig. 11 demonstrates 

 this, but the similarity of our younger brain is much more surprising. The resemblance 



* St. George Mivart " On the JElnrmdin" Proc. Zool. Soc. London, 1882, p. 516. 



