THE DIPNOI. 



171 



the primitive aortic arches are metamorphosed : but it may 

 be said, generally, that the first has disappeared ; the second 

 supplies an internal branchia developed upon the hyoidean 

 arch ; the third gives off the anterior carotid artery, and 

 supplies neither internal nor external branchia ; the foiirth 

 supplies only the first external branchia ; the fifth and sixth 

 supply both internal and external branchise; while the 

 seventh is connected only with an internal branchia. The 

 pulmonary artery seems originally to have been given off" 

 from an eighth aortic arch. 



It is a remarkable circumstance, that while the Dipnoi 

 present, in so many respects, a transition between the piscine 

 and the amphibian types of structure, the spinal column 

 and the limbs should be not only piscine, but more nearly 

 related to those of the most ancient Crossopterygian 

 Ganoids than to those of any other fishes. 



