The Rodentia, although generally presenting a low grade of development, are a very specialised and distinct group. The position here assigned to them would accord with apparent relationships with the Ungulates, through the Elephant on the one hand and the extinct _Typotherium_ on the other. In the present state of the fauna of the earth, the Carnivora form a very distinct order, though naturally subdivided into two groups, the members of the one being more typical, while those of the other (the _Pinnipedia_) are aberrant, having the whole of their organisation specially modified for living habitually in the water. The Insectivora comprise various lowly organised and generalised forms, exhibiting considerable divergence of character, and apparently connected through transitional extinct species with the Carnivora. As no other order can claim the family _Galeopithecidæ_, it is placed here, but rather for convenience than for any other consideration, since it has but little if any relationship with any of the other members. Its isolated position is indicated by assigning it a distinct subordinal rank. The Chiroptera have always been placed near the Insectivora; but they are really a highly specialised group, as much isolated from all other mammals by the modification of their anterior limbs in adaptation to aerial locomotion, as the Cetacea and the Sirenia, by the absence of hind limbs, are specially adapted for an aquatic life. Lastly, the Primates, which in any natural system must be placed at the head of the series, are divisible into two very distinct groups—one containing the various forms of Lemurs (Lemuroidea), and the other the Monkeys and Man (Anthropoidea). Whether the Lemuroidea should form part of the Primates (according to the traditional view), or a distinct order altogether removed from it, is as yet an undetermined question, for both sides of which there is much to be said. There can, however, be no doubt that the Anthropoidea form a perfectly natural group, presenting a series of tolerably regular gradations from the Marmosets (_Hapale_) to Man. Certain breaks in the series, however, enable us to divide it into five distinct families:—_Hapalidæ_ or Marmosets: _Cebidæ_ or American Monkeys, with three premolar teeth on each side of each jaw; _Cercopithecidæ_, containing the majority of Old-world Monkeys; _Simiidæ_, consisting of the genera _Hylobates_, _Simia_, _Gorilla_, and _Anthropopithecus_, the true Man-like Apes; and, lastly, _Hominidæ_, containing the genus _Homo_ alone. Subclass I. PROTOTHERIA. Order i. MONOTREMATA—Monotremes. Fam. 1. _Ornithorhynchidæ_—Duck-bill. 2. _Echidnidæ_—Spiny Anteater. Group. =MULTITUBERCULATA.=[25] Fam. 1. =Plagiaulacidæ=—Plagiaulax. 2. =Polymastodontidæ=—Polymastodon. 3. =Tritylodontidæ=—Tritylodon. Subclass II. METATHERIA. Order ii. MARSUPIALIA—Marsupials.