Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/860

Rh 796 P O P O 215 steamers which entered Oporto (no vessel, owing to the danger ous bar, comes into this port to call simply) 153 were British. History. The history of Oporto dates from an early period. Before the Roman invasion, under the name of Gaia, or Cago, it was a town with a good trade ; the Alani subsequently founded a city on the opposite or northern bank, calling it Castrum Novum. About 540 A. D. the Goths under Leovogild obtained possession of the northern district, who yielded place in 716 to the Moors under Abdul Hassan, who then conqiiered the whole of that region. The Christians, however, again gained possession by the overthrow of the Moors, when it became the key of their position for the long period during which the latter held sway in the southern provinces of Portugal. The Moors once more became its masters for a short period, till in 1092 it was brought by Dom Alfonso Frederico finally under Christian domination. The town is renowned also in English military annals from the duke of Wellington s famous passage of the Douro in its immediate neighbourhood, close to where the fine bridge of the northern railway now spans the river, by which he surprised and put to flight Soult s army, capturing the city on the 11 tli May 1809. It sustained a severe siege in 1833 during the civil war headed by Dom Miguel, and was bravely defended by Dom Pedro with 7500 men, but with the loss of 16,000 of its inhabitants. See Commercial Reports for 1878-79 ; Rrprtrt of Wine Committee of House of Commons in 1879 ; Crawfurd, Portugal, Old and Xew, 1872. (H. O. F.) OPOSSUM. The animals to which this name is applied are the only non- Australian members of the Marsupials or pouched animals, being found throughout the greater part of the continent of America, from the United States to Patagonia, the number of species being largest in the more tropical parts (see MAMMALIA Marsupialia, vol. xv. p. 380). They form the family Didelj)hyidse, distinguished from the other Marsupial families by their equally deve loped hind-toes, their nailless but fully opposable hallux ? and by their dentition, its formula being if c pm in, total 50, a number only exceeded among heterodont Mammalia by the Australian Myrmecobius fasciatus. The peculiarity in the mode of succession of these teeth has been explained in the article referred to, where also (p. 378) a figure of the teeth may be seen. 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 their great toes so fully opposable to the other digits as to constitute a functionally perfect posterior pair of &quot;hands.&quot; These oppos able great toes are without nail or claw, but their tips are expanded into broad flat pads, which are no doubt of the greatest use to such a climbing animal as an opossum. On the anterior limbs all the five digits are provided with long sharp claws, and the pollex or thumb is but little opposable. Their numerous teeth are covered with minute sharply- pointed cusps, with which to crush the insects on which they feed, for the opossums seem to take in South America the place in the economy of nature filled in other countries by the true Insectivora, the hedgehogs, moles, and shrews. The family consists of two well-recognized genera only, viz., Diddphys, containing all the members of the family, with the exception of the Yapock, a curious animal which forms by itself the second genus, Chironectes, and is dis tinguished from all other opossums by its webbed feet, non-tuberculated soles, aiid peculiar coloration. Its ground colour is light grey, with four or five sharply-contrasted brown bands passing across its head and back, giving it a very peculiar mottled appearance. It is almost wholly aquatic in its habits, living on small fish, crustaceans, and other water animals ; its range extends from Guatemala to southern Brazil. The other genus of opossums, Didelphys, is an extremely heterogeneous one, and has been split up into several groups, some of which perhaps ought also to be recognized as genera. The first of these consists of three or four lar^e dull-coloured long- and coarse-haired species, with perfect marsupial pouches, large leafy ears, and greatly-developed muscular ridges on their skulls. The best known of these, and indeed of all the family, is the Virginian Opossum, Didelphys virfjiniana, an animal spread over all temperate North America ; it is extremely common, being even found living in the towns, where it acts as a scavenger by night, retiring for shelter by day upon the roofs of the houses 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 is from fourteen to seventeen days. Another very similar species is found in central and tropical South America, and is known as the Crab-eating Opossum (D. cancrivora) The second ^ group, or sub-genus, named Metachirus, con tains 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 pouches 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. The accompanying woodcut represents Lord Derby s Opossum (D. derliana] carrying its young in this manner. Didelphys derbiana. The third group is Micoureus (Grymseomys of Burmeister), differing only from Metachirus by the comparatively 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 house-mouse, of a bright-red colour, which is found as far north as central Mexicq, and extends thence right down to the south of Brazil. The last sub-genus contains three or four wonder fully shrew-like species, of very small size, with short, hairy, and non-prehensile tails, not half the length of the trunk, and with wholly unridged skulls, The most striking mem ber of the group is the Three-striped Opossum (D. tristriata], from Brazil, which is of a reddish-grey colour, with three clearly-defined deep-black bands down its back, very much as in some of the striped mice of Africa. This sub-genus has been named Hemiurus or &quot;half -tail&quot; by Geoffrey Saint -Hilaire (Microdelphys of Burmeister), and should perhaps be allowed full generic rank. The numerous fossil remains referable to species of the Didcl- phyidse are of special interest as showing some of the connecting links in geographical distribution between the opossums and the Australian Marsupials, now so widely and absolutely separated. They consist of the bones of a considerable number of species from the Eocene and Early Miocene deposits of central France, one or two Eocene species having also been found in southern England These ancient opossums have been separated generically from Di delphys on account of certain differences in the relative sizes of the lower premolars, but as nearly the whole of the species have been formed on lower jaws only, of which some hundreds have been found, it is impossible to judge how far these differences are correlated with other dental or osteological characters. In the opinion of Dr Filhol, who has devoted considerable attention to the subject, the fossils themselves represent two genera, Pcrathcrium, containing