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 coal, iron, steel, machinery and textiles. The total yearly value of the foreign trade exceeds £5,000,000.

The history of Oporto dates from an early period. Before the Roman invasion, under the name of Portus Cale, Gaia or Cago, it was a town on the south bank of the Douro with a good trade; the Alani subsequently founded a city on the north bank, calling it Castrum Novum. About 540 the Visigoths under Leovigild obtained possession, but yielded place in 716 to the Moors. The Christians, however, recaptured Oporto in 997, and it became the capital of the counts of Portucalia for part of the period during which the Moors ruled in the southern provinces of Portugal. (See : History.) The Moors once more became its masters for a short period, till in 1092 it was brought finally under Christian domination. The citizens rebelled in 1628 against an unpopular tax, in 1661 for a similar reason, in 1757 against the wine monopoly, and in 1808 against the French. The town is renowned in British military annals from the duke of Wellington's passage of the Douro, by which he surprised and put to flight the French army under Marshal Soult, capturing the city on the 12th of May 1809. Oporto sustained a severe siege in 1832–1833, being bravely defended against the Miguelites by Dom Pedro with 7000 soldiers; 16,000 of its inhabitants perished. In the constitutional crises of 1820, 1826, 1836, 1842, 1846–1847, 1891 and 1907–1908 the action of Oporto, as the capital of northern Portugal, was always of the utmost importance.  OPOSSUM, an American Indian name properly belonging to the American marsupials (other than Caenolestes), but in Australia applied to the phalangers (see ). True opossums are found throughout the greater part of America from the United States to Patagonia, the number of species being largest in the more tropical parts (see ). They form the family Didelphyidae, distinguished from other marsupial families by the equally developed hind-toes, the nailless but fully opposable first hind-toe, and by the dentition, of which the formula is i. , c. , p. , m. ; total 50. The peculiarity in the mode of succession of these teeth is explained in the article referred to. Opossums are small animals, varying from the size of a mouse to that of a large cat, with long noses, ears and tails, the latter being as a rule naked and prehensile, and with the first toe in the hind-foot so fully opposable to the other digits as to constitute a functionally perfect posterior “hand.” These opposable first toes are without nail or claw, but their tips are expanded into broad flat pads, which are of great use to these climbing animals. On the anterior limbs all the five digits are provided with long sharp claws, and the first toe is but little opposable. The numerous cheek-teeth are crowned with minute sharply-pointed cusps, with which to crush the insects on which these creatures feed, for the opossums seem to take in South America the place in the economy of nature filled in other countries by hedgehogs, moles, shrews, &c. The true opossums are typically represented by Didelphys marsupialis, a species, with several local races, ranging over the greater part of North America (except the extreme north). It is of large size, and extremely common, being even found living in towns, where it acts as a scavenger by night, retiring for shelter by day upon the roofs or into the sewers. It produces in the spring from six to sixteen young ones, which are placed by the mother in her pouch immediately after birth, and remain there until able to take care of themselves; the period of gestation being from fourteen to seventeen days. A local race found in Central and tropical South America is known as the crab-eating opossum (D. marsupialis cancrivora). The second sub-genus, or genus, Metachirus contains a considerable number of species found all over the tropical parts of the New World. They are of medium size, with short, close fur, very long, scaly and naked tails, and have less developed ridges on their skulls. They have, as a rule, no pouch in which to carry their young, and the latter therefore commonly ride on their mother's back, holding on by winding their prehensile tails round hers, as in the figure of the woolly opossum. The latter belongs to the sub-genus Philander. which is nearly allied to the last; its full title being Didelphy

(Philander) lanigera. The philander (D. [P.] philander) is closely related.



The fourth sub-genus (or genus) is Marmosa (Micourcus, or Grymacomys), differing from the two last by the smaller size of its members and by certain slight differences in the shape of their teeth. Its best-known species is the murine opossum (D. murina), no larger than a mouse, of a bright-red colour, found as far north as central Mexico, and extending thence to the south of Brazil. A second well-known species is D. cinerea, which ranges from Central America to western Brazil, Peru and Bolivia. Yet another group (Peramys) is represented by numerous shrew-like species, of very small size, with short, hairy and non-prehensile tails, not half the length of the trunk, and unridged skulls. The most striking member of the group is the Three-striped Opossum (D. americana) from Brazil, which is of a reddish grey colour, with three clearly-defined deep-black bands down its back, as in some of the striped mice of Africa. D. dimidiata, D. nudicaudata, D. domestica, D. unistriata and several other South American species belong to this group. Lastly we have the Chiloe Island opossum (D. gliroides), alone representing the sub-genus Dromiciops, which is most nearly allied to Marmosa, but differs from all other opossums by the short furry ears, thick hairy tail, doubly swollen auditory bulla, short canines and peculiarly formed and situated incisors.

Whatever difference of opinion there may be as to the right of the above-mentioned groups to generic separation from the typical Didelphys, there can be none as to the distinctness of the water-opossum (Chironectes minimus), which differs from all the other members of the family by its fully webbed feet, and the dark-brown transverse bands across the body (see ).

See O. Thomas, Catalogue of Marsupialia and Monotremata (British Museum, 1888); “On Micoureus griseus, with the Description of a New Genus and Species of Didelphyidae,” ''Ann. Mag. Nat.'' Hist. ser. 6, vol. xiv. p. 184, and later papers in the same and other serials.

 OPPEL, CARL ALBERT (1831–1865), German palaeontologist, was born at Hohenheim in Württemberg, on the 19th of December 1831. After studying mineralogy and geology at Stuttgart, he entered the university of Tübingen, where he graduated Ph.D. in 1853. Here he came under the influence of Quenstedt and devoted his special attention to the fossils of the Jurassic system. With this object he examined in detail during 1854 and the following year the succession of strata in England, France and Germany and determined the various palaeontological stages or zones characterized by special guide-fossils, in most cases ammonites. The results of his researches were published in his great work Die Juraformation Englands, Frankreichs und des südwestlichen Deutschlands (1856–1858). In 1858 he became an assistant in the Palaeontological Museum at Munich. In 1860 he became professor of palaeontology in the university at Munich, and in 1861 director of the Palaeontological Collection. There he continued his labours on the Jurassic fauna, describing new species of crustacea, ammonites, &c. To him also we owe