I v. ] MONO TREMA TA. 43 



large in the sixth, but generally absent in the seventh 

 vertebra. 



In the Wombat, the bodies are wide and depressed. The 

 transverse processes are perforated in all ; the inferior lamella 

 of the sixth is much developed antero-posteriorly. The 

 spines of all are rather short. 



In Perameles lagotis the greater part ot the transverse pro- 

 cess of the axis is ossified separately from the rest of the 

 vertebra, and remains sometime distinct, as in the Mono- 

 tremata. In this genus, as in the other carnivorous Marsu- 

 pials, the inferior lamellae of the transverse processes of the 

 fourth, fifth, and sixth vertebrae, but especially of the latter, 

 are particularly large. 



Some species of American Opossums (as Didelphys 

 virgifiiana and its nearest allies) have the spinous processes 

 of the second, third, fourth, and fifth cervical vertebrae, very 

 high, square, and massive, and being closely applied to each 

 other by flattened surfaces, form a solid wall of bone along 

 the top of the neck. 



In both genera of Monotremata (sub-class Ornitho- 

 delphia) the cervical vertebrae are seven in number, and in 

 both the inferior arch of the atlas is completely ossified, 

 apparently from a separate centre ; but in Ornithorhynchus 

 a large bifurcated hypapophysis is developed, which is quite 

 wanting in Echidna. 



In Ornithorhynchus also all the other cervical vertebrae 

 have a single median hypapophysial spine, equally wanting 

 in Echidna. 



In both, the axis has a high compressed spine, and the 

 odontoid portion remains long distinct from the true centrum 

 of the bone. In both, the transverse processes are of auto- 

 genous formation, and remain suturally connected with the 

 remainder of the vertebra until the animal is nearly full- 





