Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/400

Rh 378 MAMMALIA [MONOTREMATA. C 21. Acetabulum not perforated. Tongue not extensile. Mucous membrane of small intestine covered with delicate, close-set trans verse folds or ridges. Tail rather short, broad, and depressed. Eyes very small. Fur close and soft. One species, 0. anatinus (Shaw), 0. paradoxus (Blum. ), the duck-billed Platypus, or &quot;Water- Mole of the colonists, entirely aquatic in habits, diving with great facility, and burrowing in the banks of rivers. It feeds on water insects, small mollusca, and worms, and inhabits Australia and Tasmania. See OKNITHOKHYNCHUS. Family ECIIIDNID.E. Cerebral hemispheres larger and well convoluted. Facial portion of skull produced into a long, tapering, tubular rostrum, at the end of which the anterior nares are situated. Kami of mandible slender, styliform. Opening of mouth small, and placed below the extremity of the rostrum. No laterally placed horny teeth, though the palate and tongue are furnished with spines. Tongue very long, vermiform, slender, and protractile. Lining membrane of small intestine villons, but without transverse folds. Feet not webbed, but with long strong claws fitted for scratching and bur rowing. The hinder feet with the ends of the toes turned outwards and backwards in the ordinary position of the animal when on the ground. Tail very short. Acetabulum with a large perforation, as in Birds. Calcaneal spur and gland of the male much smaller than in Ornithorhynchus. Fur intermixed with strong, sharp-pointed spines. Terrestrial and fossorial in habits, feeding exclusively on ants, and recalling in the structure of the mouth and various other parts relating to the peculiar mode of life the true Anteaters of the order Edentata. Recent discoveries have shown that there are two distinct forms of this family, which may even be considered of generic value. Echidna (Cuvier, 1797) or Tackyglossus (Illiger, 1811). 1 Claws five on each foot. Rostrum moderately developed and straight. Vertebrae : C 7, D 16, L 3, S 3, C 12. Tongue tapering at the tip, the spines restricted to the basal portion. The best-known species is E. aculeata (Shaw), found in Australia and Tasmania. The specimens from the latter locality, with longer fur almost conceal ing the spines, have been separated specifically under the name of E. sctosa (Guv.). Another species, E. lawesii ( Earn say), has lately been discovered in southern New Guinea. See ECHIDNA. Acanthoglossus (Gervais). Ungual phalanges and claws present only on the three middle digits of both fore and hind feet. Ros trum much elongated and curved downwards at the end. Vertebra : C 7, D 17, L 4, S3, C 1 2. Tongue somewhat spoon-shaped near the tip, and armed on its dorsal surface with three rows of recurved spines. One species : A. bruijnii (Peters and Doria) (fig. 22), from I uj. 2 2. Acanthoglossus bruijnii. From Gervais. the mountainous regions of the northern part of New Guinea ; con siderably larger than E. aculeata. The external characters and osteology of this animal, one of the most interesting of recent zoological discoveries, have been fully described and figured by Gervais (Osteographic des Mmwtremes, Paris, 1878). Among bones of extinct Marsupials of Pleistocene age from the Darling Downs, Mr Kreift found a portion of a humerus of an Echidna, considerably larger than the existing Australian species, which he has named E. owcni. t Notwithstanding the strong pre sumption of antiquity of the Monotrematous type, derived from its inferiority of structure, no fossil remains oil earlier date, re ferable to it, or connecting it with the lower vertebrates on the one hand and the higher mammals on the other, have yet been discovered. 1 The latter na-ne is oft used now, under the impression tliat Echidna is preoccupied by Forster (1778) for a genus of Pisces; but, as that was not characterized in a recognizable manner, the author even omitting to name the species for which it was intended, it is now generally considered a synonym for Murtena (see Giinther s Catalogue of Fishes ), and is scarcely sufficient to bar a name so universally acknowledged and so deeply rooted in mammalian literature. Merrem s genus Echidna (Rep(ilia) is of later date, viz., 1820. a Annals and Mtfj. Xnt. Hist., 1868, vol. i. (ser. iv.)p. 11 3. SUBCLASS METATHERIA ou DIDELPHIA. Although the great diversity in external form, in many anatomical characters, and iu mode of life of various animals of this section might lead to their division inti&amp;gt; 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, the limits of which are therefore equivalent to that of tho subclass. The more essentially distinctive characters have been already pointed out (p. 371). These may be more fully stated as follows. The brain is generally small in proportion to the size of the animal, and the surface folding of the cerebral hemi spheres, though well marked in the larger species, is never very complex in character, and is absent in the smaller and medium- sized 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 con tinued forward above the corpus callosum, which is of very small size. The anterior commissure i, on the other hand, greatly developed. 3 There are always true teeth, implanted in the usual manner in both jaws, and divisible, according to their posi tion 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 each side of each jaw. The chief peculiarity in the dentition lies, however, in the mode of succession. There is no vertical displacement and succession of the teeth, except in the case of a single tooth on each 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. 23), and is the only tooth comparable to those FIG. 23. Teeth of Upper .Taw of Opossum (Didtlfhys virginianci), all of which are unchanged, except the third premolar, the pl.ice of which is occupied in the young animal by a molaiiform tooth, represented in the figure below the line of the other teeth. called &quot;milk teeth&quot; in the diphyodont Eutheria. In some cases (as in Hypsiprymnus) this tooth retains its place and function until the animal has nearly, if not quite, attained its full stature, and 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 most rudimentary in form and size, being shed or absorbed before any of the other teeth have cut the gum, and therefore quite function- less. It must further be noted that there are some Marsupials, as the Wombat, Koala, Myrmecobitis, and the Dasyures, in which no such milk tooth, even in a rudi mentary state, has yet been discovered, possibly in some cases from want of materials for observation at the right stage of development. 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 vertebras is always nineteen, although there are some apparent exceptions caused by the last lumbar being 3 W. H. Flower, &quot; On the Commissures of the Cerebral Hemispheres of the Marsupial/a,&quot; &o., Phil. Trans., 1865, p. 633.