In most of the Marsupials which have a long tail, this appendage is subject to pressure on some part of the under surface. In the Kanga roo (fig. 103,) this must obviously take place to a considerable degree when the tail is used as a fifth extremity, to aid in supporting or pro pelling the body. In the Potoroos and Bandi coots the tail also transmits to the ground part of the superincumbent pressure of the body by its under surface, when the animal is erect, but it is not used as a crutch in locomotion as in the Kangaroos. In the Phalangers and Opossums the tail is pre hensile, and the vessels situated at the under surface are liable to compression when the animal hangs suspended by the tail. To protect these vessels, therefore, as well as to afford additional attachment to the muscles which execute the various r movements for which the tail is adapted iu the above mentioned Marsupials,V-shaped bones, or inferior arches (hamapophyses) are developed, of various forms and sizes, and are placed opposite the articulations 1 of the vertebra, a situation which is analo gous to that of the superior arches in the sacral region of the spine in Birds, and in. the dorsal region of the spine in the Chelonian Reptiles. The two crura of the sub-vertebral arch embrace and defend the bloodvessels, and the spinous process continued from their point of union presents a variety of forms in different genera.
In Cook's Phalanger I find the ham apophyses commence between the second and third caudal tebrae, increase in length to the fourth, and then progressively diminish to the end of the tail ; the penultimate and mate presenting a permanent ration of the lateral moieties, and an absence of the spine (fig. 104.) In the Virginian Opossum and Vulpine Phalanger they are ple, about a quarter of an inch in length where longest, and directed obliquely forwards, and diminish in size as they approach the mity of the tail. In the Potoroos the extremity of the long anterior spines is dilated and produced both backwards and forwards; the posterior smaller ones become panded laterally, and give off similar but shorter processes from each side, whereby the base of support is extended. In the Great Kangaroo the spine of the first subvertebral arch only is simple and elongated, the extremities of the others are expanded, and in some jut out into four obtuse cesses, two at the sides, and two at the anterior and posterior . faces. In a carefully prepared skeleton of Maeropus Bennettii in the Museum of the Zoological Society, I found these inferior spines want ing between the last vertebr.e of the tail. In the Petaurists, Phaseogales, and Da syures, where the tail acts as a balancing pole, or serves, from the long and thick hair with which it is clothed, as a portable blanket to keep the nose and extremities warm during sleep, the subvertebral arches are also present, but in less number and of stnal'er relative size. They
are here principally subservient to the attach ment of muscles, their more mechanical office of defending the caudal vessels from pressure not being required.
2'horax.—The ribs consist of thirteen pairs, except in the Wombat, which has fifteen, and l'etaurists, which have twelve; the first is the shortest, and, except in some of the Petau rists, the broadest. In the Pet. macrurus the fifth, sixth, or seventh are the broadest, and the ribs generally have, both in this species and in Pet. a more compressed form than in the other 11larsupials ; but this character does not exist in Petaurus Tagnanoides. In the Great Kangaroo they are very slender arid rounded, except at the sternal extremities, which are flattened for the attachment of the cartilages. In this species and the Bush Kangaroo, the seven anterior pairs of ribs articulate directly with the sternum. The cartilages of the six false pairs are long and bent towards the ster num, hut do not join it, nor are they confluent, but have a gliding motion one over the other.
In the Myrrnecohius there are eight pairs of true ribs ; the two last pairs are floating ribs. In the Opossum there are seven pairs of true and six which may be regarded as cosi& noth%e. In the Petaurist six pairs out of the twelve, and in the %Vombat six pairs only out of the fifteen, reach the sternum (fig. 105).
The sternum consists of a succession of elon gated bones, generally six in number, but in the Petourns Taguanoides five, and in the Wombat four.
The first bone, or manubrium sterni, is the largest, and presents in many species a triangu lar shape from the expansion of its anterior part, and sometimes a rhomboidal figure. A strong keel or longitudinal process is given off in many species from the middle of its inferior or outer surface ; the side next the cavity of the chest is smooth and slightly concave. In the NVom bat, Phalangers, and others, the keel is pro duced anteriorly into a strong process, against of which tire clavicles abut : the first pair of ribs join the produced anterior angles of the manubrium.
In the Dasyures, Opossums, Phalangers, and Petaurists, the manubrium is compressed arid elongated, and the clavicles are joined to a process continued from its anterior extremity ; the small clavicles of the Kangaroo have a similar connexion.
The cartilages of the true ribs (which fre quently become ossified in old Marsupials) are articulated as usual to the interspaces of the sternal bones; the last of these supports a broad flat cartilage.