MAMMALIA. 758 MAMMALIA. Boon reached its liiuits on that side, and wc are now far nearer a natural classification than we have ever been previously. It is generally agreed now that the class Mammalia is verj' naturally divisible into two suholasses. according to the ar- rangement of the reproductive orpins and the relations l>etveen mother and offspring. The pri- mary divisions ought to be based on such char- acters, for it is in just those characters that the mammals stand out in sharpest contrast to the other vertebrates. These two subclasses are: ( 1 ) Prototheria, or Ornithodelphia — mammals without teats. (2) Theria, or Eutheria — mammals with teats, arranged according to two plans: (a) Metatheria, or Didelphia. (b) Eutheria (proper) , or 5Ionodel])hia. In the I'rototheria the female reproductive organs resenilile those of birds. There is no vagina, the oviducts opening separately into the cloaca, and the uteri are merely enlargements of the oviducts. The eggs contain much yolk,, are supplied with albumen and covered with a .shell as in reptiles and birds. They are laid in a nest at the end of a burrow and are hatched very soon, possibly in twenty-four liours. The mam- mary glands have no teats, any part of the longi- tudinal depression along the centre of the abdo- men (the i)rimitive pouch) serving that purpose whm the sucking mouth of tlic young one is applied to it. This group contains but one order, Monotremata. The Theria are mammals in which the mam- mary glands are pro-ided with teats, and there is ft more or less perfect allantoic development. These two facts, especially, have led zoiilogists to join into a single group the Metathcria and Eutheria. hitherto regarded as separate groups of co.'irdinate rank with I'rototheria. These groups are thus distinguished: In the Metathcria (Didelphia of early authors) the oviducts of the female are united in a longer or shorter part of their length, and there is no cloaca, thoiigli the urogenital and anal openings are surroimded by a common sphincter muscle; each ovitluct possesses a well-formed uterus. There are thus two uteri, whence the name 'Didelphia.' The eggs of the Melalberia have little yolk and no shell. They lie in the uteri without becoming vei-y firmly united to the walls, in some species not at all: in others, however, there is formed quite a distinct placenta. (The foregoing groups have been called 'Aplaccnt-alia.') But the young remain in the uteri only a short time; even in the giant kangaroo, the largest of the subclass, they are bora on the 39th day. When born, they are transferred by the mother to her pouch, where each (me is placed on a teat, to which it clings automatically. This interest- ing subclass, like the preceding, contains but one order, the Marsupial ia (q.v. ). The Eutheria comprise all the remaining (higher) mammals. In these the marsupium is absent, the vagina is fully formed and single, and there is only one uterus. The egg of the Mono- delphian mammal is very minute, almost with- out yolk, and iH'comes closely connected with the wall of the uterus as soon as it enters that organ. (Hence this groiip has been called 'Pla- centalia.') The young develop there in closest union with the mother until their various organs are all distinctly formed and they are in condition to take milk from the mammary glands by their own efforts; but. even after birth they are tended and protected by the mother. The vast majority of the 10.000 knowTi species of mammals belong in this subclass. It has proved the dominant. the successful type. Australasia is the only land the placentals do not possess, and there geo- graphical barriers have served to protect the marsupials from being crushed liy their success- ful rivals. The classification of Eutheria is not easy to agree upon, but the following outline is that of Parker and Haswell (see Classification OF AXIMALS) : SUBCLASS EUTHEBIA. Section I. Metatheria. J Order Jlarsupialia — Marsupials. H Section II. Kulherin Vera — Higher mammals. Order 1. Edentata — Edentates. Onler 2. Cetacea — Whales, dolphins, etc. _ Older 3. Sirenia — ilanatees. dugongs, etc. ■ Order 4. Ungiilata — Hoofed mammals. " Order .5. Camivora — Beasts of prey. Order 6. Rodentia — Rodents, or gnawers. Order 7. Insectivora — Insectivores. Order 8. Chiroptera — Bats. Order 9. Primates — ilan, apes, and lemurs. Still more recently (11101), Beddard has pub- lished an arrangement which includes certain ex- tinct orders and places them in a succession "which is believed to indicate their phylogenetic relationships in a better manner than was possible to his predecessors, as follows: Marsu- pialia, Edentata, Ganodonta, Ungiilata. Sirenia, Cetacea, Carnivora, Creodonta, Rodentia. Tillo- dontia, Insectivora. Chiroptera, Primates (qq.v. ). There is no great difl'erence between these two schemes. FcssiL M.MMALIA. Most of the mammalian ty])es of modern times can be traced back to early Tertiary time, but before that period only scat- tered synthetic or generalized types are knowni. In the late Paleozoic (Permian) and the earliest Mesozoic (Lower Triassic) rocks arc found re- mains of the so-called Theriodontia and C4onipho- dontia (see Reptile), which possess characters remarkably mammalian in expression. These late highly specialized reptiles may be the root- stocks from which the most primitive ancestral forms of mammals of the Triassic and .Jurassic were evolved. The mammals of the early ileso- zoic were of very small size, not larger than rats, and, as they are known chiefly by their jaws and teeth alone, which are of very primitive type, they can with dilliculty be assigned to any of the later groups of the class. Some of these fragraental remains cannot be definitely distin- guished as either reptiles or mammals, for their teeth show characters which are possessed by both the anomodont reptiles and the undoubted primitive mammals. Such forms are Jlicro- lestes of the European Upper Trias. Tritylodon of the Ivaroo formation of South Africa, and Ste- reognathus of the English Lower .Jurassic. The principal genera of undoubted prototherian mam- mals (primitive jMonotremata) are Plagiaulax, Bolodon.Ctenacodon. Ptilodus. and Polymastodon. These are known only by small jaws containing sharp incisor teeth, three or four clo.sely set cut- ting teeth (premolars) . of which the fourth is the largest, and the outer edges of which are marked by oblique furrows (see illustration under Duck- bill), and finally two to four small molars. These molars are multituberculate, and hence
