280 



MARSUPIALIA. 



ing between the last nine vertebrae of the 

 tail. In the Petaurists, Phascogales, 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 smaller 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. 



Thorax. The ribs consist of thirteen pairs, 

 except in the Wombat, which has fifteen, and 

 Petaurists, which have twelve; the first is the 

 shortest, and, except in some of the Petau- 

 rists, the broadest. In the Pet. mucrurus the 

 fifth, sixth, or seventh are the broadest, and 

 the ribs generally have, both in this species and 



in Pet. sciureus, a more compressed form than 

 in the other Marsupials ; but this character 

 does not exist in Petaurus Taguanoides. In the 

 Great Kangaroo they are very slender and 

 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, but do not join it, nor are they confluent, 

 but have a gliding motion one over the other. 



In the Myrmecobius there are eight pairs of 

 true ribs ; the two last pairs are floating ribs. 

 In the Opossum there are seven pairs of true 

 ribs, and six which may be regarded as costte 

 nothff. In the Petaurist six pairs out of the 

 twelve, and in the Wombat six pairs only out 

 of the fifteen, reach the sternum (Jig. 105). 



Fig. 105. 



Phascolomys fusca. 



The sternum consists of a succession of elon- 

 gated bones, generally six in number, but in 

 the Petaurus 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 Wom- 

 bat, Phalangers, and others, the keel is pro- 

 duced anteriorly into a strong process, against 

 the sides of which the 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 

 and 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. 



Of the Pectoral Extremities. The scapula 

 varies in form in the different Marsupials. 

 In the Petaurists it forms a scalene triangle, 

 with the glenoid cavity at the convergence of 

 the two longest sides. 



In the Wombat it presents a remarkably re- 

 gular oblong quadrate figure, the neck being 

 produced from the lower half of the anterior 

 margin, and the outer surface being traversed 

 diagonally by the spine, which in this species 

 gradually rises to a full inch above the plane of 

 the scapula, and terminates in a long narrow 

 compressed acromion arching over the neck to 

 reach the clavicle. 



In the Koala (Jig. 106), the superior costa 

 does not run parallel with the inferior, but re- 

 cedes from it as it advances forwards, and then 

 passes down, forming an obtuse angle, and with 

 a gentle concave curvature to the neck of the 

 scapula ; a small process extends from the 

 middle of this curvature. In the Potoroo, the 

 upper costa is at first parallel with the lower, but 

 this parallel part is much shorter ; the remainder 

 describes a sigmoid flexure as it approaches the 

 neck of the scapula. 



In the Great Kangaroo, the Perameles, Pha- 

 langers, Opossums, and Dasyures, the whole 

 upper costa of the scapula describes a sigmoid 



