the Siamang (H. syndactylus), which presents so many peculiarities of form, that a brief description of it may not be unacceptable as a contribution to our knowledge of the cerebral anatomy of the Primates. As the cast taken from the interior of the skull may be presumed to represent in all its proportions the brain which filled that cavity, I shall in future speak of it as such, and when reference is made to the brain of allied forms, casts of the interior of the skulls of adult male specimens are intended, unless otherwise specified.

The most obvious characteristics of the general form of this brain (see Fig. 1) are its breadth and great depression. It even exceeds in flatness that of some of the lower apes, and presents a marked contrast to the globular brain of its ally, the Orang. The length of the cerebrum is 3 inches, its greatest breadth 2.5 inches, its height 1.9 inches. Its outline, when seen from above, presents a five-sided figure, with the angles rounded off; truncated behind, with lateral boundaries nearly parallel for more than half their length, and afterwards rapidly converging to the anterior pointed apex. This extremity is prolonged downwards and forwards, and ends in the well developed olfactory bulbs, which project slightly beyond the cerebrum, occupying the cavity of which the floor is formed by the wide cribriform plates of the ethmoid bone, and which has not that extremely contracted aperture seen in the Cynocephali and other lower apes. In the depressed and pointed form of the frontal lobes, and the position of the olfactory bulbs, the Siamang departs widely from the more anthropomorphous apes, in all of which the cerebral hemispheres are so developed in the frontal region as to cover the olfactories. But, on comparing this brain with that of a Macaque or Baboon, the under surfaces of the anterior lobes appear less excavated, and consequently contain a larger amount of cerebral substance. The temporal lobes are well prolonged downwards, flattened on their external surface, and when seen from below appear narrow, and standing wide apart from each other.

But the most striking peculiarity of the brain is the backward projection of the cerebellum beyond the level of the cerebral hemispheres, a circumstance, as far as I am aware, unknown in any other ape, either of the Old or New World. On looking from above, both the posterior surface of the vermis, and the rounded edges of the la

it is in contact, so plastic that it may be pulled out, without injury, from any underhanging depressions in the skull cavity, and yet so elastic that it will immediately regain its exact form. From this a mould is made in plaster in as many pieces as may be necessary, according to the complexity of the form of the object, and out of this mould any number of casts are taken in the usual manner. These casts give a perfect and most convenient working model of the general form of the brain, which, owing to the peculiar softness of the cerebral tissues, can rarely be preserved in the actual specimen. Their utility has especially been insisted on by Gratiolet.

teral lobes stand out in an unmistakeable manner, the latter to the extent of nearly 2-10ths of an inch. In the cast, the impression of the cavities, in which the lateral sinuses are lodged, give an apparent increase to this projection, but this is readily distinguished from that of the lobes themselves. The median portion or vermis is lodged in a slight vallecula as in the human cerebellum, and although not quite extending to the level of the lateral lobes, is more prominent than in the higher anthropoid apes.

This peculiarity of the Siamang's brain is due to two causesfirstly, the large development of the cerebellum; secondly, and I shall afterwards show, mainly, to the actual shortness of the posterior or occipital lobe of the cerebrum.

It is difficult to find a method of easy application for estimating the proportionate size of the cerebellum to the cerebrum, but Ï think that the relative breadth of the two portions of the encephalon (which may be ascertained from the skulls alone) affords a measure, approximatively, at least, accurate. The following table, embracing examples of the principal genera of the Old World apes, drawn up upon this plan, shows that, as we descend from the anthropoid forms, the proportionate size of the cerebellum decreases :

It will be seen by this, that the Siamang is grouped with the three large anthropoid apes, which have all very well developed cerebella, but it does not exceed any of them in this respect.

On examining and measuring a large series of skulls and brains of various apes, it appears to me that the diminution in the size of the cerebellum affects its length far less than its breadth, and is chiefly evident at the antero-lateral portions of each hemisphere. So that the horizontal angle formed by the superior margin of the petrous bone (which marks the antero-lateral attachment of the tentorium, and consequently the margin of the cerebellum) with the median line of the skull, affords a fair index of the proportionate

development of the cerebellum. In the Chimpanzee this angle is 55°, in the Siamang the same, in the Entellus monkey 50°, in the Chacma baboon 43°. Diminution of the cerebellum in this region would have no influence upon its backward prominence as related to that of the cerebral lobes, so I conclude that the size of the cerebellum, although it may have some effect, is not largely concerned in the varying extent to which it is covered in the posterior direction by the cerebrum, in the different forms of Primates.

A comparison of the cast, Fig. 1., with that of the brain of either a higher or lower form of ape, will show that the second proposition is well founded. In the brain of most Primates, the line bounding inferiorly the temporal and occipital lobes, commencing at the apex of the former, passes backwards, ascends rapidly, then sweeps backwards again, making a wide curve with the concavity downwards, before terminating at the hinder end of the occipital lobe. In the Siamang this line (marking the tentorial plane) is almost straight, rising at an angle of 40° to the long axis of the brain, and cutting off, as it were, abruptly, that portion of the hemisphere so extensively developed in the Cynocephali and Chrysothrix (Figs. 2 and 3). And it will be seen by referring to the figures, that it is not alone to the obliquity of the inferior surface that the smallness of the occipital lobe is due; there is also a marked falling away of the upper contour of the brain in the occipital region.

Not being able to deduce anything further from the cast as to the relative proportions of the several parts of the Siamang's brain, and having no actual specimen available for examination, I have had recourse to the recent brain of a young Gibbon of a different species, dissected last summer (and which is now under the care of my friend Dr. Rolleston, in the Museum of the University of Oxford), for some further illustrative facts. This brain has a more globular, and less depressed form than that of the Siamang; this may be partly due to younger age, and partly to specific difference. The cerebellum is very large, and barely covered by the cerebral lobes. The convolutions present the same general characters as those described in the brain of H. leuciscus by Gratiolet, but I am able to add that on the inner face of the hemisphere (not figured or described by Gratiolet) it exhibits the anthropic character of an internal perpendicular fissure joining the calcarine, observed also in some Semnopitheci, but not in any lower form of Old World apes. The convolutions and sulci of the external surface exhibit a similar high grade of development, as shown by, 1. Their multiplicity, taking into account the small size of the brain. 2. Their want of bilateral symmetry. 3. The superficial position of the superior annectent gyrus, which completely intercepts the external perpendicular fissure on the right side, and partially on the left. The limit between the parietal and occipital lobes is indicated by this fissure, and its backward position shows the reduction of the size of the last.

FIG. 2. Cast of the interior of the skull of the Chacma Baboon (Cynocephalus porcarius), adult male. The dotted line indicates the outline of the cavity which lodges the olfactory lobes.

FIG. 3. Cast of the interior of the skull of the Saimiri or Squirrel Monkey (Chrysothrix sciureus), adult.

The figures are all from specimens in the Museum of the Royal College of Surgeons, and are of the natural size.

In order to practise the mode of estimating the length of the posterior as compared with the antero-median portions of the cerebrum, which I have described elsewhere,* a horizontal section was made through the right hemisphere, when it was ascertained that the portion anterior to the convex posterior border of the hippocampus major, including the corpus striatum and optic thalamus, measured 1.7 in., while the portion behind this point, including the posterior cornu and hippocampus minor, was but 7 in.; the proportion being as 100 to 41. On comparing this figure with those in the table given in the appendix to the paper just referred to, it will be seen that it is inferior to that of any true ape yet measured; the figures in other genera ranging from 47 (Semnopithecus), to 62 (Hapale), the proportion in the Human brain averaging about 53. I have no doubt, from the external form of the lobe, that in the Siamang it would be even less than in the Gibbon.

The above mentioned external characters are all corroborated in a brain of Hylobates lar, in the Museum of the Royal College of Surgeons, and an inspection of the skulls of several members of the genus points to the same conclusion, namely, that a very great reduction of the occipital lobe of the cerebrum is one of the most marked characters of the brain of Hylobates, although it is not in any other species at present examined carried to so great an extent as in the Siamang. As the Gibbons take rank, by virtue of all their structural affinities, among the higher members of the order to which they belong, we see in this fact a conclusive illustration of Gratiolet's remark, to the effect that the large development of a particular character which distinguishes an elevated group, cannot always be selected as a sign of elevation among the individual members of the group.†

"On the posterior lobe of the Cerebrum of the Quadrumana."-Phil. Trans. 1862. The genus Colobus is not referred to anywhere above, as the brain is at present unknown, but I should infer, from the form of the skull of C. vellerosus, that it is closely allied to Semnopithecus, though in the shortness of the posterior lobes approaching even nearer to Hylobates.