50 



COMPARATIVE ANATOMY 



cervical vertebrae may become firmly fused together (Cetacea). 

 The distal parts of the transverse processes, representing rudi- 

 mentary ribs, are perforated by the vertebrarterial canal (p. 48) : 

 in Monotremes these remain distinct at any rate for a long time. 



FIG. 38. Archceopteryx lithographica. From the Solenhofen slates (Jurassic.) 

 After Dames, from the specimen in the Berlin Museum. 



The atlas and axis essentially resemble those of Birds, but the 

 differentiation of the vertebral column into regions characterised 

 by difference of form is much more sharply marked than in any 

 other Vertebrates. There are as a rule seven cervical vertebra ; 

 Bradypus, however, possesses eight to nine, and Tamandua bivit- 

 tata eight, while in Manatus and Cholcepus there are only six. 



