50 CERVICAL VERTEBRAE IN THE MAMMALIA. [CH. iv. 



In both genera of MONOTREMATA (sub-class Ornitho- 

 delphid] 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 

 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- 

 grown (see Fig. 5, p. 25) ; that of the axis is still distinct in 

 an adult Ornithorhynchus. Though in this respect they 

 present an approximation to the Sauropsida (Reptiles and 

 Birds), they differ from that group, inasmuch as there is not 

 a gradual transition from these autogenous transverse pro- 

 cesses of the neck (or cervical ribs, as they may be con- 

 sidered) into the thoracic ribs, for in the seventh vertebra 

 the costal element is much smaller than in the others, 

 indicative of a very marked separation of neck from thorax, 

 not seen in the Sauropsida. The vertebrarterial canal in 

 Echidna bruijni is very wide, and passes through all the 

 vertebrae from the second to the seventh. 



