398 PROF. OWEN ON THE OSTEOLOGY 
Phalangers and Opossums the tail is prehensile, 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 movements for which the tail is adapted in the 
above-mentioned Marsupialia, V-shaped bones, or inferior arches (hemapophyses), are 
developed, of various forms and sizes, and are placed opposite the articulations of the 
vertebrz, a situation which is analogous to that of the superior arches in the sacral re- 
gion of the spine in Birds, and in the dorsal region of the spine in the Chelonian Rep- 
tiles. The two crura of the subvertebral arch embrace and defend the blood-vessels, 
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 hemapophyses commence between the second and 
third caudal vertebre, increase in length to the fourth, and then progressively diminish 
to the end of the tail, the penultimate and antepenultimate presenting a permanent 
separation of the lateral moieties, and an absence of the spine. In the Virginian and 
Vulpine Opossum and Phalangers they are simple ; about a quarter of an inch in length 
where longest, directed obliquely forwards, and diminish in size as they approach the 
extremity of the tail. In the Potoroos the extremity of the long anterior spines is di- 
lated and produced both backwards and forwards ; the posterior smaller ones become 
expanded 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 processes, two at the sides and two at the anterior and posterior surfaces. In 
a carefully-prepared skeleton of Macropus Bennettii I found these inferior spines wanting 
between the last nine vertebre of the tail. In the Petaurists, Phascogales and Dasyures, 
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 rela- 
tive size ; they are here principally subservient to the attachment of muscles, their more 
mechanical office of defending the caudal vessels from pressure not being required. 
Of the Thorax.—Of the ribs, which, with the exception of the Wombat and Petaurists, 
are thirteen pairs, the first is the shortest, and, except in some of the Petaurists, the 
broadest. In the Pet. macrourus 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 Petawrus Taguanoides. 
In the Great Kangaroo they are very slender and rounded, except at the sternal extre- 
mities, 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 sternum, but do not 
