546 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



4. Ligamenti carotidinae normales (ligamenti ottleyi): Both anterior dorsal 

 radices aortae (dorsal carotids) atrophy, but remain as the ligaments of 

 Ottley and enter the hypapoph3'sial canal. This is a condition of rare 

 occurrence and has been observed in Bucorvus and Rhopodytes. This 

 condition represents the culmination of the bicarotid evolution except 

 for the unicarotid arrangements. 



Unicarotid arrangements 



1. Conjuncto-carotidinae normales: A single carotid artery enters the hj^japo- 



physial canal, but this is supplied by a pair of vessels of equal size from the 

 common carotids of both left and right sides. This arrangement is quite 

 common among the Ciconiiformes. 



2. Conjuncto-carotidinae abnormales: The same as in 1, above, except that the 



basal vessel is reduced in diameter on one side. This is the first level in the 

 modification of the conjugate carotid arrangement and is found in the 

 flamingos and herons. 



3. Ligamentum carotidinae-conjuncti: As in 2, above, or further modified except 



that the lumen of the reduced vessel is not complete and the distal portion 

 of the basal vessel is reduced to a ligament. This condition is found to 

 exist at two levels of atrophy: (1) second level modification results from 

 atrophy at the anterior end of one of the basal vessels, but with a lumen for 

 nearly half of its length, and (2) third level modification results from com- 

 plete closure of the basal vessel with retention of a ligamentous vestige. 

 This ligament may be entire or partial. The degree of resorption appears to 

 vary in different species. 



4. Laevo-carotidinae or dextro-carotidinae normales: The same as in 3, above, 



except that there is no remaining vestige of the ligamentous connection from 

 the opposite side. This is the fourth level modification of the unicarotid 

 arrangement and is commonly found in many orders of birds. 



5. Laevo-carotidinae or dextro-carotidinae infranormales: The same as 4, above, 



except that the functional carotid is superficial and does not enter the 

 hypapophysial canal. This has been reported in a single passerine genus, 

 Orthonyx. 



6. Ligamentum unicarotidinae (ligamentum ottleyi): The culmination of the 



unicarotid evolution results in atrophy of the single dorsal carotid artery. 

 This may be at either of two levels: (1) retention of the ligamentous vestige, 

 or (2) partial or complete resorption of the ligament. In such a case, the 

 vertebrals and superficial cervical arteries will take over the function of 

 supplying the blood to the head. 



To simplify and codify the above classification of carotid arrange- 

 ments, the following scheme is suggested; it may serve to indicate 

 more nearly those close similarities and gi-oss dissimilarities which 

 may be presumed to exist and to indicate which orders of birds may be 

 undergoing important anatomical evolution: 



Class A 



A-1 Bicarotidinae normales. 



A-2-d Bicarotidinae abnormales: right vessel superficial. 



A-2-S Bicarotidinae abnormales: left vessel superficial. 



A-3 Bicarotidinae infranormales. 



A-4 Ligamenti carotidinae normales (ligamenti ottleyi). 



