SKELETON OF MAKSUPIALIA. 



214 



V-shaped bones, or haemal arches, are developed, of various forms 

 and sizes, and are placed beneath the articulations of the vertebrae, 

 a situation which is analogous to that of the neural arches in the 

 sacral region of the spine in Birds, and in the dorsal region of 

 the spine in the Chelonian Reptiles. The two crura of the 

 hfemal arch embrace and defend the bloodvessels, and the spinous 

 process continued from their point of union presents a variety of 

 forms in different genera. In Cook's Phalanger the hasmapo- 

 physes commence between the second and third caudal 

 vertebrae, increase in length to the fourth, and then 

 progressively diminish to the end of the tail ; the 

 penultimate and antepenultimate presenting a per- 

 manent separation of the lateral moieties, and an 

 absence of the spine, fig. 214. In the Great Kan- 

 garoo the spine of the first haemal arch only is simple 

 and elongated, the extremities of the others are ex- 

 panded, and in some jut out into four obtuse pro- 

 cesses, two at the sides, and two at the anterior and 

 posterior surfaces. 



The cervical vertebrie, seven in number in all Mar- 

 supials, show usually to the last the circumscription of 

 the vertebrarterial foramen by confluence of a short 

 pleurapophysis, fig. 216, pi, with di- and met-apophy- 

 ses : but I have seen the pleurapophyses still unan- 

 chylosed in a full-grown Perameles. In Dasyures, 

 Opossums, Phalangers, and Perameles, the seventh 

 cervical has the diapophysis only : in the Kangaroos 

 both atlas and dentata may have the transverse pro- 

 cess merely grooved by the vertebral arteries : in the 

 Koala and Wombat the atlas presents only the per- 

 foration on each side of the superior arch. In the 

 Perameles and some other Marsupials, the neurapo- 

 physes of the atlas, fig. 216, n, are distinct from the '^caSf^ 

 hypapophysis, fig. 215, h, as well as from their proper PhTSfr. 

 centrum, the odontoid, fig. 216, c a. In the Koala 

 and Wombat the hypapophysis remains cartilaginous, and the 

 lower part of the vertebral ring is completed, in the 

 skeleton, by dried gristly substance, fig. 216. In the 

 Petaurists, Kangaroos, and Potoroos, the atlas is 

 completed below by an extension of ossification from 

 the neur apophyses into the cartilaginous hypapophy- 

 sis, simulating the body, and the ring of the vertebra is for a long 

 time interrupted by a longitudinal fissure in the middle line, the 



215 



Atlas of Pera- 

 meles lagotis. 



