MAUSUPIALIA. 



2?? 



arteries; and in the Koala and Wombat the 

 atlas presents only the perforation on each side 

 of the superior arch. In the Perameles and 

 some other Marsupials, as the Cayopollin, an 

 affinity to the cold-blooded Ovipara is mani- 

 fested in the structure of the atlas (Jig. 98), 

 which exhibits a permanent se- 

 paration of the neurapophyses 

 or superior laminae from the 

 centre or body below. In the 

 Koala and Wombat the body 

 tne at ^ as remains perma- 

 nently cartilaginous, and the 

 lower part of the vertebral ring 

 by dried gristly substance 



Fig. 98. 



Atlas of Peru- 

 meles'lagotis, 



is completed 



(Jig. 90). In the Petaurists, Kangaroos, 



Fig. 99. 



Atlas, axis, and third cervical vertebra, Koala. 



and Potoroos, the atlas is completed below 

 by an extension of ossification from the 

 neurapophyses into the cartilaginous nucleus 

 representing the body, and the ring of the 

 vertebra is for a long time interrupted by a 

 longitudinal fissure in the middle line, the 

 breadth of which diminishes with age. This 

 fissure is represented in figures of the atlas of 

 a Potoroo and Kangaroo, given by Pander and 

 D'Alton, ( Beutelthiere,Jig. c, plates iii. &vii.); 

 but in some of the skeletons of these Marsu- 

 pials examined by me I find the ring com- 

 pleted and the fissure obliterated. In all the 

 Marsupials the spine of the dentata is well 

 developed both in the vertical and longitudinal 

 directions, but most so in the Virginian and 

 Crab-eating Opossums, (fig. 100), 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 (Jig. 101), which equals in height and 



Fig. 100. 



Fig. 101. 



Vertebra dentata, 

 Didelpliys Virginiana. 



Third cervical vertebra, 

 Didelp/iys Virginiana. 



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. I know of no other 

 mammiferous genus which presents the same 

 structure: in the Armadillos the corresponding 

 spines are largely developed, but they are 

 anchylosed together. In the Orang the cervical 

 spines are very long and strong, but have the 

 ordinary sub-cylindrical rounded form. Tyson, 

 who has described and figured the above struc- 

 ture of the cervical vertebrae in his anatomy of 

 the Opossum, conjectures that it is given to 

 this arboreal animal in order " that there might 

 be no danger of its breaking its neck should it 

 happen to fall to the ground by chance or de- 

 sign:" but this teleological conjecture is inva- 

 lidated by the fact that the Phalangers, Petau- 

 rists, Koala, and other arboreal Marsupials 

 present the usual structure of the five posterior 

 cervical vertebrae, the spines of which are much 

 smaller and weaker than that of the dentata, 

 and in the Phalangers and Petaurists almost 

 obsolete. These do not require the neck to be 

 strengthened to aid in overcoming the struggles 

 of a resisting prey. I observe in the Pha- 

 langista Cookii that the superior flattened 

 arches of the five last cervical vertebrae bear 

 a ridge on each side of the spine having 

 the same direction and form, and nearly the 

 same size. The structure of the transverse 

 processes of the cervical vertebrae, in the Opos- 

 sum, is adapted to the strengthening and fix- 

 ation of this part of the vertebral column : 

 they are expanded nearly in the axis of the 

 spine, but obliquely, so that the posterior part 

 of one transverse process overlaps the anterior 

 part of the succeeding. This structure is ex- 

 hibited in a slighter degree in the cervical ver- 

 tebrae of the Dasyures, Phalangers, and Great 

 Kangaroo. In the Petaurists, Potoroos, Wom- 

 bat, and Koala, the direction and simpler form 

 of the transvetse processes allows of greater 

 freedom of lateral motion. In the Koala and 

 Wombat a short obtuse process is given off 

 from the under part of the transverse process 

 of the sixth cervical vertebra. In the Poto- 

 roos, Kangaroos, Petaurists, Phalangers, Opos- 

 sums, and Dasyures, this process is remarkably 

 expanded in the direction of the axis of the 

 spine. In the Bandicoots corresponding pro- 

 cesses are observed progressively increasing in 

 size, on the fourth, fifth, and sixth cervical 

 vertebrae. 



The number of the dorsal vertebrae is greatest 

 in the Wombat, where it is fifteen, correspond- 

 ing with the number of pairs of ribs : it is 

 least in the Petaurists, which have twelve 

 dorsal vertebrae.* In all the other genera there 



* In the skeletons both of the Pet. macrurus and 

 Pet. sciweut in the Museum of the College of Sur- 

 geons there are twelve pairs of ribs ; but in the 

 Pet. macrurus the succeeding vertebra has a short 

 transverse process on each side, the extremity of 

 which has the appearance of having supported a 

 costal appendage. Cuvier, however, assigns but 

 twelve dorsal vertebrae to this species in his table. 

 Lee. d'Anat. Comp. 2d edit. p. IbO. 



