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Show 1892.] MUSCULAR A N A T O M Y O F A U L A C O D U S . 527 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 any 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 very 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 patagonical and all other Rodents whose brains I have examined. As, however, the hemispheres 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 m y 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 backwardly-directed furrow, which joins the Sylvian fissure, thus cutting off a small triangular piece of brain about 2 m m . 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 m m . in length (a, in fig. 2 ) ; separated from this by a space of about 5 m m . is the anterior part of it, which is even less extensive. 1 " Notes on the Anatomy of Bolichotis patagonica," P. Z. S. 1891, p. 236. 36* |