544 PROCEEDrNGS OF THE NATIONAL MUSEUM vol. i04 



used as an index in specific levels of evolution. The following scheme 

 is proposed for the classification of the thoracic arteries in birds: 

 Type 1: attachment to the pectoral stem lateral to the axillary. 

 Type 2: attachment to the subclavian between the axillary and coracoid. 

 Type 3: attachment to the subclavian at the base of the coracoid. 

 Type 4: attachment to the subclavian, but with a common root for both the 



coracoid and thoracic. 

 Type 5: attachment to the subclavian medial to both the axillary and coracoid. 

 Type 6: two separate thoracic arteries are present; the primary thoracic is the 

 same as type 1 above, while the secondary thoracic is the same as 

 type 3 or type 4 above. 

 The medial migration of the thoracic artery appears to have some 

 phylogenetic significance as yet not understood. 



Arrangements of dorsal carotid arteries 



Insofar as the early embryonic stages in the development of the 

 dorsal carotids are concerned, all birds may be considered to be 

 bicarotid, but during subsequent stages in development many parts 

 are deleted or functionally modified in an orderly sequence of events. 

 As a result, higher-level deletions may be regarded as significant as 

 indices of more recent derivation or of higher levels of species evolu- 

 tion, and with particular respect to the aortic arch system. Most 

 recent evolutionary changes in the aortic arch system are related to the 

 adult condition of the anterior dorsal radices aortae or dorsal carotid 

 arteries. 



Since the bicarotid condition is more primitive than the unicarotid 

 condition, the former is to be considered as representing a lower level 

 in the evolution of the system, and any variation of the unicarotid 

 condition may be considered to represent an advance over the bicarotid 

 condition. Within each of the main groups, however, there are certain 

 special arrangements or modifications which may be regarded to be of 

 additional value in determining relative positions within a family or 

 order with respect to the evolution of the organ system. 



A description of each of the known and anticipated arrangements 

 of the dorsal carotid arteries is essential, and to clarify the classifica- 

 tion of the carotid arrangement it is proposed that the bicarotid condi- 

 tion be referred to as Class A and the unicarotid condition be referred 

 to as Class B. In addition, certain numerical values are assigned to 

 the variations within each of these classes, and these numerical 

 values may serve as indices of levels of evolution or specialization. 

 Further, the letters d and s serve to indicate right or left side. 



Bicarotid arrangements 



1. Bicarotidinae normales: Both dorsal carotids enter the hypapophysial canal 

 and pass anteriorly to the head without fusing. This arrangement is found 

 in most orders of birds and is to be regarded as the basic arrangement. 



