smooth, or with few and simple convolutions in a very small proportion, composed of the largest members, of the group. The mammals so characterized constitute the subclass LISSENCEPHALA' (fig. 5).

The third leading modification of the Mammalian cerebrum is such an increase in its relative size, that it extends over more or less of the cerebellum; and generally more or less over the olfactory lobes. Save in very few exceptional cases of the smaller and inferior forms of Quadrumana (fig. 6), the superficies is folded into more or less numerous gyri or convolutions (fig. 7),—whence the name GYRENCEPHALA, which I propose for the third subclass of Mammalia'.

In Man the brain presents an ascensive step in development, higher and more strongly marked than that by which the preceding subclass was distinguished from the one below it. Not only do the cerebral hemispheres overlap the olfactory lobes and cerebellum, but they extend in advance of the

1 λισσός, smooth; ἐγκέφαλος, brain.

2 yupów, to wind about; éyképaλos, brain.

Fig. 8.

one and further back than the other (figs. 8 & 9). Their posterior development is so marked that anthropotomists have assigned to that part the character and name of a 'third lobe:' it is peculiar and common to the genus Homo: equally peculiar is the 'posterior horn of the lateral ventricle' and the 'hippocampus minor,' which characterize the hind lobe of each hemisphere. The superficial grey matter of the cerebrum, through the number and depth of the convolutions, attains its maximum of extent in Man.

Peculiar mental

powers are associated with this highest form of brain, and their consequences wonderfully illustrate the value of the cerebral character; according to my estimate of which, I am led to regard the genus Homo as not merely a representative of a distinct order, but of a distinct subclass, of the Mammalia, for which I propose the name

of ARCHENCEPHALA (fig. 9)1.

1 dpxw, to overrule; èyképaλos, brain.

With this preliminary definition of the organic characters, which appear to guide to a conception of the most natural primary groups of the class MAMMALIA, I next proceed to define the groups of secondary importance, or the subdivisions of the foregoing subclasses.

The Lyencephalous Mammalia are unguiculate: some have the optic lobes' simple, others partly subdivided, or complicated by accessory ganglions, the lobes being then called 'bigeminal bodies.'

The LYENCEPHALA with simple optic lobes are ' edentulous' or without calcified teeth, and are devoid of external ears, scrotum, nipples, and marsupial pouch: they are true 'testiconda:' they have a coracoid bone extending from the scapula to the sternum, and also an epicoracoid and episternum as in Lizards: they are unguiculate and pentadactyle, with a supplementary tarsal bone supporting a perforated spur in the male. The order so characterized is called 'MONOTREMATA,' in reference to the single excretory and generative outlet, which, however, is by no means peculiar to them among Mammalia. It includes two genera-Echidna and Ornithorhynchus. Of the first, the species are terrestrial, insectivorous, chiefly myrmecophagous, having the beak-like slender jaws, and long cylindrical tongue of the true anteaters; but they are covered, like the hedgehog, with spines. Of the second genus, the species are aquatic, with a flattened beak, like that of a duck, which is used in the anserine manner to extract insects and worms from the mud: but they are clothed with a close fine fur like that of a mole, whence the name 'duck-mole' by which these anomalous quadrupeds are commonly known to the colonists. Both genera of Monotremes are strictly limited to Australia and Tasmania.

The LYENCEPHELA with divided optic lobes, forming the corpora bigemina' and 'quadrigemina' of anthropotomists, have teeth, and with rare exceptions, the three kinds, viz. incisors, canines, and molars. They are called MARSUPIALIA, because they are distinguished by a peculiar pouch or

duplicature of the abdominal integument, which in the males is everted, forming a pendulous bag, and in the females is inverted, forming a hidden pouch containing the nipples and usually sheltering the young for a certain period after their birth: they have the marsupial bones in common with the Monotremes; a much varied dentition, especially as regards the number of incisors, but usually including 4 true molars ; and never more than 3 premolars1 (fig. 2): the angle of the lower jaw (ib. a) is more or less inverted2.

With the exception of one genus, Didelphys, which is American, and another genus Cuscus, which is Malayan, all the known existing Marsupials belong to Australia, Tasmania, and New Guinea. The grazing and browsing Kangaroos are rarely seen abroad in full daylight, save in dark rainy weather. Most of the Marsupialia are nocturnal. Zoological wanderers in Australia, viewing its plains and scanning its scrubs by broad daylight, are struck by the seeming absence of mammalian life; but during the brief twilight and dawn, or by the light of the moon, numerous forms are seen to emerge from their hiding-places and illustrate the variety of marsupial life with which many parts of the continent abound. We may associate with their low position in the mammalian scale the prevalent habit amongst the Marsupialia of limiting the exercise of the faculties of active life to the period when they are shielded by the obscurity of night.

The premature birth of the offspring, and its transference to the tegumentary pouch, in which it remains suspended to the nipple for a period answering to that of uterine life in higher mammals, relate to the peculiarities of the climate of Australia.

The adventurous and much-enduring explorers of that continent bear uniform testimony to the want of water as the

1 Outlines of a Classification of the Marsupialia, Trans. Zool. Soc. Vol. II. 1839. 2 For other Osteological and Dental characteristics of the Marsupialia, see the paper above cited, and that On the Osteology of the Marsupialia, Trans. Zool. Soc. Vol. I. p. 379 (1838).

chief cause of their sufferings and danger. During the dry season the rivers are converted into pools, 'few and far between ;' and the drought is sometimes continued so long as to dry up these. An ordinary non-marsupial quadruped, such as the wild cat or fox, having deposited her young in the nest or burrow, would in such a climate, at the droughtiest period of her existence, be compelled to travel a hundred, perhaps two hundred miles, in order to quench her thirst. Before she could return her blind and helpless litter would have perished. By the marsupial modification the mother is enabled to carry her offspring with her in the long migrations necessitated by the scarcity of water.

With the climatal peculiarities of Australia, therefore, we may connect the peculiar modifications of those members of the mammalian class which are most widely distributed over that continent. But the principle of final causes receives more especial illustrations from the contingent particulars of the marsupial organization. The new-born Kangaroo is an inch in length, naked, blind, with very rudimental limbs and tail: in one which I examined the morning after the birth, I could discern no act of sucking: it hung, like a germ, from the end of the long nipple, and seemed unable to draw sustenance therefrom by its own efforts. The mother, accordingly, is provided with a peculiar adaptation of a muscle (cremaster) to the mammary gland, by which she can inject the milk from the nipple into the mouth of the pendulous embryo. Were the larynx of the little creature like that of the parent, the milk might probably would-enter the windpipe and cause suffocation: but the foetal larynx is cone-shaped, with the opening at the apex, which projects, as in the whale-tribe, into the back aperture of the nostrils, where it is closely embraced by the muscles of the soft palate.' The air-passage is thus completely separated from the fauces, and the injected milk passes in a divided stream on either side the base of the larynx into the oesophagus. These correlated modifications of maternal and foetal structures, designed with especial