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SOME account of the general traits of Didelphys Virginiana, its external form and covering, and dentition, may properly precede, and serve as an introduction to, the study of its bones and muscles. Among other articles upon its anatomy, physiology, and zoology, those cited below may be consulted. ¹ The present memoir is based upon original dissections and preparations, from which all the descriptions have been taken, except in a few instances where the contrary is stated; and all the illustrations were drawn by the writer from the same sources. The opossum's form is adapted to the execution of numerous and varied movements, but not fitted for great strength, sustained effort, or rapid progression. The animal is sluggish, usually moving slowly and deliberately; its fastest pace is a kind of amble, when both legs of the same side are simultaneously moved, as is also the case when it is walking more slowly; it has not been observed to trot or lope. It is plantigrade, and the body is usually carried so low as to barely clear the ground. The body is thickset and clumsily shaped; the centre of motion is behind the middle, as shown by the general contour, as well as by the trend of the spines of the vertebræ. Extension of the body, as a whole, is less conspicuous than flexion, and is not ordinarily exhibited except when the animal is hanging suspended by the tail; but the body may be bent double, or rolled in a ball, with the greatest ease, the fore and hind feet passing by each other at the middle of the belly, and the nose pointing backward, reaching the root of the tail. The faculty of bringing the fore-paws and snout into such unusual positions is undoubtedly in …