76 



THE OSTEOLOGY AND MYOLOGY 



hour-glass contraction. On the remiiining vertebrae, these processes are simply little lat- 

 eral knobs, representing, on the sides of the bones, the rudimentary zygapophyses on top. 

 From about the seventh or eighth, they take part in tlie articulation. Rudiments of both 

 articular and transverse processes may be traced to the very last bone ; but they are very 

 insignificant in size beyond the fifteenth or sixteenth, and articulation is really effected, 

 beyond the sixth vertebra, by the ends of the centrums themselves. 



In this scansorial prehensile-tailed marsupial, as in the saltatorial forms of the orders, 

 the remarkable feature of " hsemal arches " is found. The use of these singular arches is, 

 beyond a doubt, the protection of the blood vessels from undue pressure. They only occur 

 in those species in which locomotion is aided by the tail, as the kangaroo, phalangers and 

 opossums. In the present species these arches begin between the third and fourth caudals 

 and continue uninterrupted to the very last. These chevrons are perfect throughout the 

 series — not presenting the separation of the lateral moieties that occurs with the ter- 

 minal ones in some other genera. They are V-shaped, and situated directly at the artic- 

 ulation ; the proximal ones are the largest, in general, and the rest successively diminish 

 to the last ; but the second one is larger than the first, and the only one that presents a 

 large hcTmal spine, directed backward. The first half-dozen or so have decided spines in 

 the direction of the arches ; the rest no more protuberance than necessarily results from 

 the coalescence of the two crura of the arch. From first to last all are perforate, and 

 transmit vessels. Beside defending the circulation, they give leverage to the powerful 

 flexor muscles of the tail. The ingenious device of these hcemal arches affords a beauti- 

 ful illustration of exceptional development to meet special emergency. 



3. The Thorax. 



The dorsal vertebrae having been already considered, it only remains to notice, in this 



connection, the ribs and sternum (fig. 18). 



There are thirteen ribs, six of which join the ster- 

 num, and three each other, leaving four free. The 

 pleurapophyses alone are osseous ; the hsemapophy- 

 ses remain cartilaginous, or at most, had acquired 

 but little bone-earth (and that only in the upper 

 ribs) in the oldest individuals I dissected. The first 

 pleurapophysis is very shorts — less than an inch long 

 in well grown specimens — owing to the great con- 

 traction of the apex of the thorax ; very stout, 

 nearly straight, with a large tubercle, that appears 

 as the real end of the bone, the neck being short 

 and slender, set off at nearly a right angle from the 

 main shaft, and bearing a slightly enlarged head. 

 The ribs rapidly lengthen {e.g., the third is more 

 than twice as long as the first) to the sixth, seventh 

 and eighth, which are about equal to each other, and 

 longest ; and then more gradually shorten to the 

 last. Coincidently with increase of length, the 



^'a:^S:?^::t:r^:^'^r^. ^^--ve of the shaa becomes greater, as does also the 



