332 ANATOMY OF VERTEBRATES. 



in the living animal. In the Koala, fig. 227, the tail is also 

 very short. In the Ch^ropus it seems to be wanting. In one 

 species of Perameles I find eighteen caudal vertebra ; in another 

 twenty-three. In two species of Potoroo there are twenty- 

 four caudal vertebra, but the relative length of the tail differs 

 in these by one third, in consequence of the different length 

 of the bodies of the vertebra. In Hypsiprymnus ursinus there 

 are more than twenty-six caudal vertebrae. In the Great Kan- 

 garoo there are twenty-two caudal vertebrae. In Bennett's Kan- 

 garoo there are twenty-four caudal vertebrae, which are remark- 

 able for their size and strength. In the Phalangista vulpina, 

 there are twenty-one caudal vertebrae. In the Petaurus ma- 

 crurus I find twenty-eight caudal vertebrae, while in the Pet. 

 sciureus there are but twenty ; the bodies of the middle caudal 

 vertebrae in both these species are remarkably long and slender. 

 The Myrmecobius has twenty-three caudal vertebrae : in Didelpliys 

 cancrivora there are thirty-one ; in the Virginian Opossum there 

 are twenty-two caudal vertebrae. In the latter species the spinal 

 canal is continued along the first six; beyond these the neural 

 spines cease to be developed, and the body gives off, above, only 

 the two anterior and two posterior zygapophyses which are rudi- 

 mental, and no longer subservient to the mutual articulation of 

 the vertebrae. 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 vertebrae 

 they are generally the longest. In the other vertebrae a short 

 obtuse 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 ap- 

 pendage is subject to pressure on some part of the under surface. 

 In the Kangaroo, fig. 211, this must obviously take place to a 

 considerable degree when the tail is used as a fifth extremity, to 

 aid in supporting or propelling the body. In the Potoroos and 

 Bandicoots the tail also transmits to the ground part of the super- 

 incumbent 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 

 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 Marsupials/ 



