334 



ANATOMY OF VERTEBRATES. 



216 



breadth of which clmiinishes with age. In all the Marsupials the 

 spine of the dentata is well developed both in the vertical and longi- 

 tudinal directions, but most so in the Virginian and Crab-eating 

 Opossums, fig. 217, where it increases in thickness posteriorly; 

 in these species also the third, fourth, and fifth cervical vertebrae 



have their spines remarkably long and 

 thick, but progressively diminishing 

 from the third, fig. 218, which equals 

 in height and thickness, but not in 

 longitudinal extent, the spine of the 

 dentata. These spines are four-sided, 

 and being closely impacted together, 

 one behind another, must add greatly 

 to the strength, while they diminish 

 the mobility, of this part of the spine. 

 The structure of the transverse pro- 

 cesses of the cervical A^ertebrje, fig. 

 218, d, is also adapted to the strength- 

 ening and fixation of this part of the 

 vertebral column : they are expanded nearly in the axis of the 

 spine, but so that the posterior part of one transverse process 



overlaps the anterior part of 



Atlas, axis, and third cen'ical vertebra, 

 Koala. 



217 



218 



Vertebra dentata, 

 Didelphys Virginicma. 



Third cervical vertebra, 

 Didelphys Virginiana. 



the succeeding. This struc- 

 ture is exhibited in a slighter 

 degree in the cervical verte- 

 brae of the Dasyures, Pha- 

 langers, and Great Kangaroo. 

 In the Petaurists, Potoroos, 

 Wombat, and Koala, the di- 

 rection and simpler form of the 

 transverse processes allow of 

 greater freedom of lateral mo- 

 tion. In the Koala and Wombat a short obtuse process is given 

 oif from the under part of the transverse process of the sixth 

 cervical vertebrae. In the Potoroos, Kangaroos, Petaurists, Pha- 

 langers. Opossums, and Dasyures, this process is remarkably 

 expanded in the direction of the axis of the spine. In the Bandi- 

 coots corresponding processes are observed, progressively increasing 

 in size, on the fourth, fifth, and sixth cervical vertebra?. 



B. Skull — The form of the skull varies much in different 

 Marsupials, but it may be said, in general terms, to resemble an 

 elongated cone, being terminated by a vertical plane surface 

 behind, and in most of the species converging toward a point 



