1892.] MUSCULAR ANATOMY OF AULACODTJS. 52J^ 



hemispheres to Capromys ; this latter Rodent has the peculiar 

 rounded hemispheres that characterize the Porcupines. As in most 

 Rodents (and many of the lower Mammalia) the optic lobes are 

 largely exposed ; the degree to which the corpora quadrigemina are 

 exposed in Aulacodus differs from that of anj^ Rodent with which 

 I have been able to compare its brain : the difference chiefly 

 depends upon the form of the posterior margin of the hemispheres ; 

 these are very closely approximated in the middle line, and diverge 

 posteriorly at a ver}' wide angle ; the posterior boundaries of the 

 hemispheres, indeed, meet almost in the same straight line ; there is no 

 widening out of the median sulcus to form a triangular space, such 

 as is evident, for example, in Dolichotis patagonica^ and all other 

 Rodents whose brains I have examined. As, however, the hemi- 

 spheres do not come into contact with the cerebellum in the middle 

 part above, there is a space left which is occupied by the two 

 posterior lobes of the corpora quadrigemina. These two only are 

 visible and they are raised almost to the level of the hemispheres 

 themselves. As a rule, when the brain of a Rodent is viewed in 

 profile, the corpora quadrigemina are seen upon the floor of a deep 

 depression. Compare, for example, the accompanying drawing and 

 fig. 4 A of my paper upon Dolichotis quoted below. 



The cerebral hemispheres of Aulacodus are but faintly fissured. 



The Sylvian fissure is, however, well marked, though short in 

 extent; it runs on each side almost vertically upwards, its direction 

 being, indeed, rather forwards at first and then curving backwards. 

 Just in front of the Sylvian fissure at its origin is a short backwardlv- 

 directed furrow, which joins the Sylvian fissure, thus cutting off' a 

 small triangular piece of brain about 2 mm. in length ; this perhaps 

 represents the Island of Reil. The Sylvian fissure of Aulacodus is 

 much better marked than it is in either Myopotamus or Capromys, 

 in both of which the fissure is barely discernible. 



The upper surface of the brain is but little marked by sulci ; 

 I have already pointed out that there is not an obvious relation 

 between the size of the animal and the complexity of its brain- 

 convolutions in the Rodentia. The Beaver with its nearly smooth 

 brain is perhaps the most striking instance ; and this example is 

 additionally remarkable from the fact that aquatic mammalia seem, 

 as a general rule, to have more richly convoluted brains than their 

 purely terrestrial relations. The only fissure is the longitudinal 

 fissure corresponding, I imagine, to that termed by Sir Richard Owen 

 " lambdoidal " ; the only fissures upon the brains of Myopotamus 

 and Capromys were the same, which is so strongly developed in 

 Dolichotis, Ccelogenys, and Dasyprocta. In Aulacodus this fissure 

 does not run, as it does in Dolichotis, continuously from one end of 

 the hemisphere to the other. There is a short fissure on each side, 

 7 mm. in length («, in fig. 2) ; separated from this by a space 

 of about 5 mm. is the anterior part of it, which is even less 

 extensive. 



1 " Notes on the Anatomy of Boliahotis patagonica," P. Z. S. 1891, p. 236. 



36* 



