MARSUPIALIA.J MAMMALIA 37<> modified into a sacral vertebra. The number of pairs of ribs is nearly always thirteen. The tympanic bone remains permanently distinct. The carotid canal perforates the basi-sphenoid. The lacrymal foramen is situated upon or external to the anterior margin of the orbit, and there are generally large vacuities in the bony palate. The angle of the mandible is (except in Tarsipes) more or less inflected. The hyoid bones have always a peculiar form, consisting of a small, more or less lozenge-shaped basi-hyal, broad cerato-hyals, with the remainder of the anterior arch usually unossified, and stout, somewhat compressed thyro-hyals. There are two anterior vence cavte, 1 into each of which a " vena azygos " enters. In the male the testes are always contained in a scrotum which is suspended by a narrow pedicle to the abdomen in front of the penis. The vasa deferentia open into a complete and continuous urethra, which is also the passage by which the urine escapes from the bladder, and is perfectly distinct from the passage for the freces, although the anus and the termination of the urethro-sexual canal are embraced by the same sphincter muscle. The glans is often bifurcated anteriorly. In the female the oviducts never unite to form a common cavity or uterus, but open separately into the vagina, which at least for part of its course is double. During the very short period in which the embryo is contained in the uterus, its nourishment seems to be derived from the umbilical vesicle, the allantoic vessels not reaching the surface of the chorion to form a true placenta. The mammae vary much in number, but are always abdominal in position, have long teats, and in most of the species are more or less enclosed in a fold of the integument forming a pouch or marsupium, though in some this is entirely wanting, and the newly-born, blind, naked, and helpless young, attached by their mouths to the teat, are merely concealed and protected by the hairy covering of the mother s abdomen. In this stage of their existence they are fed by milk injected into their stomach by the contraction of the muscles covering the mammary gland, the respiratory organs being modified temporarily, much as they are permanently in the Cetacea.) the elongated upper part of the larynx projecting into the posterior nares, and so maintaining a free communication between the lungs and the external surface independently of the mouth and gullet, thus averting the danger of suffocation while the milk is passing down the latter passage. The existing species of Marsupials are, with the excep tion of one family (the Didelphidx), limited in geographical distribution to the Australian region, forming the chief mammalian fauna of Australia, New Guinea, and some of the adjacent islands. The Didetphidx are almost purely neotropical, one or two species ranging northwards into the Nearctic region. Fossil remains of members of this family have also been found in Europe in strata of the Eocene and early Miocene period. In dividing the Marsupials into minor groups, it may be observed that one of the most obvious distinctive characters among them is derived from the form and arrangement of the teeth. In certain species, as the Opossums, Dasyures, and Tliylacines, the incisors are numerous, small, and subequal in size and the canines large, as in the typical placental Carnivores (fig. 24 ; compare with that of Lion, vol. xiv. p. 680). To these the term "polyprotodont " is applied, and they are all more or less carnivorous in their habits. In others the central incisors are very prominent, and the lateral incisors and canines absent or subordinate in function (fig. 25). These are called " diprotodont," and they are all wholly or in great part vegetable feeders. 1 Except in Bdideus brcviceps (Forbes, Proc. Zool. Soc., 1881, p. 188). In one group of these, the Wombats, there are but two incisors above and the same number below ; but all the others, including the Kangaroos, Koalas, and Fhalangers, FIG. 24. Front View of Skull of Dasyimis ursinus, showing polyprotodont and carnivorous dentition (Proc. Geol. Sue., 18G8, p. 313). have two incisors below and as many as six above, three on each side, but of these the first or central pair is the most fully developed.* Though this division is extremely convenient, a difficulty in accepting it as marking a radical separation of the order into two pri mary stocks is caused l>y the Peramelidse, which combine a poly protodont form of dentition with a pecu liar structure of the hind feet, so exactly resembling that of .some of the best- marked diprotodonts, ns the Kangaroos, that it is difficult to believe that it can have been developed independently. Tak- ^ ^_^ ^ Qf Sku]] of Koa)a (phaic<>! lllg VariOUS COmbma- arctos cinereus), showing diprotodont and hcr- t ions Of Characters Wvorona dentition (I roc. Geol. Soc., 1868, p. 313j. into consideration, the existing Marsupials readily group themselves into six very natural families, the characters of which can be thus defined : A. Teeth rooted. a. Polyprotodont. Incisors numerous, small, subequal. Canines larger than the incisors. Molars with sharp tubercles. a. Incisors . Hind feet with the four outer toes sub- equal, distinct, and a well-developed opposable hallux. 1. Didclphidse. /3. Incisors f. Hind feet with four outer toes distinct, subequal. Hallux small or rudimentary; rarely opposable. 2. Dasyuridas. y. Incisors |. Hind feet long, narrow. Fourth toe larger than all the others. Hallux rudimentary or absent. Second and third toes very slender and united in a common integument (syndactylous). 3. Pcramclidae. b. Diprotodont. Incisors f Central upper and lower incisors large and cutting. Canines absent or small. Molars with bluntly tuberculated or transversely ridged crowns. Hind feet syndactylous. o. Hind limbs disproportionately large, with feet as in Peramclidaz. 4. Macropodidse. 0. Hind limbs not disproportionately large. Feet broad, with four subequal outer toes, and a largo
opposable Lai lux. 5. I hular.gistidte.