CHAPTER VI THE SUBCLASS METATHERIA OR DIDELPHIA General Characters.-The Metatheria or Didelphia are represented at present by numerous species, presenting great diversities of general appearance, structure, and habits, although all united by many essential anatomical and physiological characters, which, taken altogether, give them an intermediate position between the Prototheria and the Eutheria. Although the striking differences in external form, in many anatomical characters, and in mode of life of various animals of this section might lead to their division into groups equivalent to the orders of the Eutheria, it is more convenient on the whole to adhere to the usual custom of treating them all as forming one order called MARSUPIALIA,1 the limits of which are therefore equivalent to that of the subclass. The more essentially distinctive characters are as follows. In the structure of the brain and the presence of epipubic bones they agree with the Prototheria, while in the structure of the earbones and the shoulder-girdle and the presence of teats on the mammary glands they resemble the Eutheria, the reproductive organs belonging to neither one nor the other type, but having a special character representing an intermediate grade of development. The ureters open into the base of the bladder. The oviducts are differentiated into uterine and Fallopian portions, and open into a long and distinct vagina, quite separate from the cystic urethra. The penis is large, but its crura are not directly attached to the ischia. The spongy body has a large bifurcated bulb. young are born in an exceedingly rudimentary condition, and are never nourished by means of an allantoic placenta, but are transferred to the nipple of the mother, to which they remain firmly The 1 For the detailed characters of all the genera and species of Marsupials the reader should consult the British Museum Catalogue of Marsupialia and Monotremata, by Oldfield Thomas, 1888. attached for a considerable time, nourished by the milk injected into the mouth by compression of the muscle covering the mammary gland. They are therefore the most typically mammalian of the whole class. The nipples are nearly always concealed in a fold of the abdominal integument or "pouch" (marsupium) which serves to support and protect the young in their early helpless condition. Entering more fully into the characters of the subclass, which are also those of the order Marsupialia, it may be observed that the brain is generally small in proportion to the size of the animal, and the surface-folding of the cerebral hemispheres, though well marked in the larger species, is never very complex in character, and is absent in the medium-sized and smaller species. The arrangement of the folding of the inner wall of the cerebrum differs essentially from that of all known Eutheria, the hippocampal fissure being continued forward above the corpus callosum, which is of very, small size. The anterior commissure is, on the other hand, greatly developed. The teeth are always divisible, according to their position and form, into incisors, canines, premolars, and molars; but they vary much in number and character in the different families. Except in the genus Phascolomys, the number of incisors in the upper and lower jaws is never equal. The true molars are very generally four in number on either side of each jaw. The chief peculiarity in the dentition lies, however, in the mode of succession. Thus there is no vertical displacement and succession of the teeth, except in the case of a single tooth on either side of each jaw, which is always the hindermost of the premolar series, and is preceded by a tooth having more or less of the characters of a true molar (see Fig. 34); this deciduous tooth being the only one comparable to the "milk-teeth" of the diphyodont Eutheria. In some cases (as in Potorous) this tooth retains its place and function until the animal has nearly, if not quite, attained its full stature, and FIG. 34.-Teeth of upper jaw of Opossum (Didelphys marsupialis), all of which are unchanged, except the last premolar, the place of which is occupied in the young animal by a molariform tooth, represented in the figure below the line of the other teeth. is not shed and replaced by its successor until after all the other teeth of the permanent series, including the posterior molars, are fully in place and use. In others, as the Thylacine, it is very rudimentary in form and size, being shed or absorbed before any of the other teeth have cut the gum, and therefore quite functionless. It must further be noted that there are some Marsupials, as the Wombat, Myrmecobius, and the Dasyures, in which no such milk-tooth, even in a rudimentary state, has yet been discovered, possibly in some cases from want of materials for observation at the right stage of development. enters. Epipubic or marsupial bones are present in both sexes of nearly all species. In one genus alone, Thylacinus, they are not ossified. The number of dorso-lumbar vetebræ is always nineteen, although there are some apparent exceptions caused by the last lumbar being 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 basisphenoid. The lachrymal 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 vena cavæ,1 into each of which a vena azygos" 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 fæces, 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. The mammæ vary much in number, but are always abdominal in position, having 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 1 Except in Petaurus (Belideus) breviceps (Forbes, Proc. Zool. Soc. 1881, p. 188). independently of the mouth and gullet, thus averting the danger of suffocation while the milk is passing down the latter passage. Distribution. The existing species of Marsupials are, with the FIG. 35.-Front view of skull of Sarcophilus ursinus, showing polyprotodont and carnivorous dentition (Quart. Journ. Geol. Soc. vol. xxiv. p. 313). exception of one family (the Didelphyidae), limited in geographical distribution to the Australasian region, forming the chief mammalian fauna of Australia, phyida are almost purely Neotropical, one or two species ranging northwards into the Nearctic region. Fossil re mains of members of this family have also been found in Europe and America in strata of the Eocene and early Miocene periods; and it is probable that at least many of the polyprotodont Mesozoic mammals. noticed in Chapter IV. are referable to the Marsupialia. Classification. In dividing the Marsupials into minor groups, it may be observed that one of the most obvious FIG. 36.-Front view of skull of Koala (Phas colarctus cinereus), showing diprotodont and herbivorous dentition (Quart. Journ. Geol. Soc. vol. xxiv. p. 313). distinctive characters among them is derived from the form and arrangement of the teeth. 1 Including the transitional Austro-Malayan region. In certain species, as the Opossums, Dasyures, and Thylacine, the incisors are numerous, small, and subequal in size, and the canines large, as in the typical placental Carnivores (Fig. 35). 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. 36). These are called "diprotodont," and they are all wholly or in great part vegetable feeders. 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 Phalangers, have two functional 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. Some hesitation has frequently been expressed as to whether the Polyprotodont and Diprotodont types are entitled to constitute distinct primary groups, owing to the presence of syndactylism among the Peramelida in the former, as well as in the latter; but if Mr. O. Thomas is right in regarding this feature as acquired independently in the two groups we may safely adopt such a division. Taking various combinations into consideration, the existing Marsupials readily group themselves into six very natural families, the leading characters of which may be summarised as follows: Order MARSUPIALIA. A. POLYPROTODONTIA.—Incisors numerous, small, subequal. Canines larger than the incisors. Molars with sharp cusps. 5 a. Incisors Hind feet with the four outer toes subequal, distinct, and a well-developed opposable hallux. Didelphyida. B. Incisors. Hind feet with four outer toes distinct. Hallux small or rudimentary, rarely opposable. Dasyurida. Hind feet long and narrow. Fourth toe (4—5) Incisors B. DIPROTODONTIA.-Incisors not exceeding 3, usually 3, but occasionally Central (first) upper and lower incisors large and cutting. Upper canines generally, and lower invariably, absent or small. Molars with bluntly tuberculated or transversely ridged crowns. a. Teeth with persistent pulps. Incisors, large, scalpriform, with enamel on the outer surface only. No canines. Hind feet with four subequal outer toes, partially syndactylous, and with rudimentary hallux. Phascolomyida. |