137 



})lete in all but the last three, which consist only of the body. 

 There are no inferior spines ; and as only the six posterior vertebraa, 

 which progressively diminish in length, extend beyond the posterior 

 aperture of the pelvis, the tail is scarcely visible in the living animal. 

 In the Koala the tail is also very short. In one species ofPerameles 

 I find eighteen caudal vertebrae, in another twenty-three. 



" In two species of Potoroo there are tvv'enty-four caudal verte- 

 bra;, but the relative length of the tall differs in these by one-third, 

 in consequence of the greater length of the bodies of the vertebrae. In 

 the GreatKangaroo there are twenty- two, while in Bennett's Kangaroo 

 there are twenty-four caudal vertebrae. In the Phalangista vulpina 

 there are twenty-one caudal vertebrae. In the Petaurus macrurus 1 

 find twenty-eight caudal vertebrae, while in the Pet. sciureus there 

 ure but twenty. I'he bodies of the middle caudal vertebrae, in both 

 these species, are remarkably long and slender. In the Dasynrus 

 Maugei I find twenty- caudal vertebrae. 



" In the Virginian Opossum there are twenty-two caudal vertebrae ; 

 the spinal canal is continued along the first six, bej-ond these the 

 superior spinous processes cease to be developed, and the body gives 

 off above only the two anterior and two posterior oblique processes, 

 Avhich are rudimental, and no longer subservient to the mutual arti- 

 culation of the vertebra?. The transverse processes are single on 

 the first five caudal vertebrae, and are nearly the breadth of the body, 

 but diminish in length from the second caudal, in which they are 

 generally tlie longest. In the other vertebrae a short obtuse trans- 

 verse process is developed at both extremities of the body on either 

 side, so that the dilated articular surfaces of the posterior caudal 

 vertebrae present a quadrate figure. 



" In most of the Marsupials, which have a long tail, this ajjpendage 

 is subject to pressure on some j)art of the under surface. In the 

 Kangaroo this must obviously take i)lace to a considerable degree 

 Avhen the tail is used as a fifth extremity to aid in supporting or jjro- 

 pelling the body. In the Potoroos and Perameles 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 Pha- 

 langers and Opossums the tail is prehensile, and the vessels situated 

 at the under surface are liable to compression A\hen the animal 

 hangs suspended by the tail. To protect these vessels, therefore, 

 as well as to afford additional attachment to the museles which execute 

 the various movements for which the tail is adapted in the above-men- 

 tioned Marsujiiata, ^'^-shaped bones, or inferior arches (Jtamapophy- 

 ses) are developed, of various forms and sizes, and are placed ojjposite 

 the articulations of the vertebrae, analogous to the situation 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 C7-ura 

 of the subvertebral arch embrace and defend the blood vessels ; and 

 the process continued from tlieir point of union ] (resents a variety 

 of forms in different genera. In the \'ii"ginian Opossum and \\x\- 

 pinc Phalangcr they are simple, about a quarter of an inch in 



