610 



PROCEEDINGS OF THE NATIONAL MUSEUM 



and ultimate loss of the basal portion of either the left or right side, 

 and the functional adult condition is either dextrocarotidinae or 

 laevocarotidinae, respectively. 



The level of ordinal evolution may be inferred by the level of 

 evolution of the dorsal carotids, at least msofar as the members of 

 the order are concerned. Within an order or family in which partial 

 or complete loss of one of the dorsal carotids has occurred, some es- 

 timate of the relative level of evolution of the species may be made. 

 A graded series in the levels of atrophic evolution of the dorsal carotid 

 arteries can be observed in several orders of birds (fig. 119). 



In the macroevolution of the dorsal carotids the levels of evolution 

 are quite simplified, but in the microevolution of these same vessels 

 there are several important steps in the reduction and ultimate 

 deletion of a vessel. 



(B-6b-sy 



(B-6a-s) 



! 



(B-5-S) (B-4-S) 



(B-3b-d) 



(&-3a-d) 



(B-2-d) 



(B-6b-d) 



I 



(B-6a-d) 



f 



(B-4-d) (B-5-d) 



\/ 



(B-3b-s) 



(B-3a-s) 



(B-2-s) 



(A-1) 



Figure 119. — Graded series or levels in the evolution of the dorsal carotid arteries in birds. 



The following orders and families, in which there is evidence of 

 some carotid artery evolution, are listed according to the carotid 

 arrangements reported for each group. Orders and families of birds 

 not included in this list either are bicarotidinae normales (A-1) or 

 the condition of the carotid arrangement has not been established. 



