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Rh of this structure upon the data thus pat before the reader upon the authority of M. Gratiolet. Leaving the task of so applying it to the reader, I shall proceed to show that the superficial position of this bridging convolution is by no means an universally present characteristic either of the human brain, or of the Orangs; and, thirdly, that it is sometimes both present and superficially visible in the brain of the Chimpanzee.

Of seven human brains at present in the University Museum, three possess this bridging convolution on both sides entirely superficial in position; in the fourth we find it wanting on one side, two spurs thrown out from the declivity of the occipital representing what is a perfectly continuous chain on the other side; in the fifth it is concealed on one side by the overhanging edge of the occipital lobes; in the sixth it does not quite reach, on the left side, the level held by the occipital and parietal lobes which it connects; in the seventh, a deep chasm is visible on both sides; but on the left the convolution, which seems to fail to bridge the fissure, does really cross it and fill it up, though at a distance of as much as an inch from the longitudinal fissure ; whilst on the right side the connecting convolution dips vertically downward, and leaves a deep valley between the occipital and parietal lobes. This seventh brain belonged to a man who, by trade a gardener, was possessed of more than an average share of intelligence, and whose brain was carefully preserved for this reason, as well as on account of its great size, and the development of its convolutions. This last of the seven brains will allow us to apply our second canon to test the value of the absence of this structure in the particular relation of superficial position as a mark of serial degradation.

But a structure which exhibits so much variability, as to conform to the rule in but three, and to diverge more or less from it in four, out of seven brains chosen at haphazard for examination, as being all at that moment which a particular museum contained, will scarcely seem to merit a high place as a zoological differentia. With reference to the "premier Pli de Passage" in the orang, a careful comparison of the relations of the parts lettered aa, in fig. 3, with the same relations in fig. 4, will show that this convolution is by no means superficial in its entire extent on the left side of that brain. And, secondly, in our second specimen of an orang' s brain, this convolution is concealed on both sides within the fissure; and the cerebral hemispheres in this specimen pre- sent, in consequence, as perfectly wave-like an opercular edge as in any other monkey. In confirmation of this, I would appeal to Tiedemann's and Wagner's figures, already referred to, as giving typical representations of an external perpendicular fissure in the brain of orang utangs, in which, according to M. Gratiolet, it should be invariably half- filled up by his "premier pli de passage."

Lastly, with reference to the chimpanzee: one specimen possesses