550 



PROCEEDINGS OF THE NATIONAL MUSEUM 



The left radix aortae usually persists thi'oughout most of its length 

 as a ligamentum aortae. 



The ducti carotici are usually functionally modified, although the 

 right ductus caroticus frequently is found as a persistent ductus in 

 several species of birds, and may be expected to occur in any species 

 of bird as an anomaly (Bhadm-i, 1939; Finn, 1891; Glenny, 1940b, 

 1944a; Mathew, 1944; Subhapradha, 1944). 



The anterior dorsal radices aortae or dorsal carotids usually lie 

 within the hypapophysial canal, although exceptions to this have been 

 reported (Beddard, 1898; Garrod, 1873; Glenny, 1954b). 



Modifications of the dorsal carotids have been discussed in the 

 previous chapter. 



Subclass ARCHAEORNITHES 



Ancestral birds, which are as yet unkiK)wn, were in all probability 

 bicarotid. In the short-necked forms these vessels may have been 

 superficial, although it is more than probable that they entered the 

 hypapophysial canal as is the case in the alligator and crocodile. 



The relatively close relationship to the crocodilians seems to be 

 confirmed by the arrangement of the cervical and thoracic arteries 

 in both birds and the alligator. The latter presents a laevo-caro- 

 tidinae normales arrangement, with the single dorsal carotid entering 

 the hypapophysial canal. 



Figure 113. — Main cervical and thoracic arteries as postulated for the Archaeornithes, 

 ventral view. (For explanation of symbols see facing page.) 



