552 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



fications was the loss of the left systemic arch with the reduction to a 

 ligament of the left radix aortae. This may have been accompanied 

 by further atrophy and reduction in the vestiges of the ducti botalli. 

 The site of junction of the vertebrals with the carotids probably under- 

 went considerable change v/ith the result that there was considerable 

 variation within the several major groups of bu-ds. 



For the most part, the bicarotid condition persisted in most of the 

 major groups of birds. But, as evolution of the aortic arch system 

 progressed, there was considerable variation in the arrangement of the 

 dorsal carotids. Durmg the process both left and right configurations 

 probably arose, but the widespread occurrence of the bicarotid and 

 laevocarotid conditions may have led to the present dominance of 

 these two main arrangement-patterns. 



Evidence that this process is continuing may be found in several 

 families of extant birds, noted below. 



Order STRUTHIONIFORMES 

 Family Struthionidae 



Carotids A-1; type A coracoid artery; type 1 thoracic artery; 

 ligamentum aortae and ligamentum botalli prominent; vertebrals 

 and superficial cervicals arise from the common carotids either 

 separately or from a common root. 



Garrod (1873) reported Struthio camelus to be bicarotidinae 

 normales. 



Eeferences: Fleming, 1926; Garrod, 1873. 



Species studied 



By Garrod 

 Struthio camelus Linn6 



By Glenny 

 Std'uthio camelus australis Gurney 



Order RHEIFORMES 

 Famih'' Rheidae 



Carotids B^-s; type A coracoid artery; typo 1 thoracic artery; 

 ligamentum aortae and ligamentum botalli present; vertebrals and 

 superficial cervicals arise from the common carotids separately but 

 in close association with each other. 



F. P. Evans (1883) stated that the right carotid is evidently pres- 

 ent, though much smaller than the left and instead of converging to 

 meet the left, which enters the hypapophysial canal, it continues 

 onwards as a superficial vessel in close association with the right 

 vagus nerve and jugular vein. 



Glenny (1943d) observed much the same condition in a rhea em- 

 bryo, but difficulty in dissection made it impossible to follow the 



