54 STRUCTURE AND CLASSIFICATION OF BIRDS 



carotids being the equivalents of the comes vagi nervi of 

 other birds. FOEBES thought that the same might be the 

 case with Leptosoma, where the carotids are unusually 

 small, and apparently bound together, not absolutely fused, 

 as also in Opisthocomus. In certain parrots the left carotid 

 artery is superficial, while the right runs in the ordinary 

 way within the vertebral canal. This is illustrated in the 

 accompanying figure (fig. 37). A final variation has been 

 observed by FOEBES in the passerine Orthoiiys, where the 

 left carotid, as in all passerines, is alone present ; but it runs 

 superficially, and there is no deep right carotid, as in the 

 parrots, just referred to. These facts, striking though they 

 are, are unfortunately of but little value in classification, or 

 at least their value is not understood. We may, however, 

 accept FOEBES'S statement that ' no passerine bird has ever 

 yet been found with more than a left carotid, and no pigeon, 

 duck, or bird of prey without two normally placed ones.' 



In all birds, as is well known, the right aortic arch ! has 

 alone persisted. It is, however, a commonplace of the text- 

 books to mention the fact that the Eaptores have often a 

 ligamentous rudiment of the left. There are occasionally 

 (perhaps individual) remains of the left arch more conspicu- 

 ous. Thus I have found a considerable tract of the left arch 

 capable of being injected, and measuring quite an inch in 

 length, in Spizcetus and in the hornbill Aceros. Cariama, 

 I may mention, has (at least sometimes) a ligament repre- 

 senting the otherwise aborted left aortic arch. 



A variation in the thigh arteries has been noted by 

 GAEEOD, who found that in some birds the ischiadic, in 

 others the femoral, w r as the most important. In Passeres the 

 neotropical Clamatores were termed by him Heteromeri, 

 since the femoral was the principal artery ; other Passeres 

 (including, however, the neotropical and clamatorial genus 

 Rupicola) Homoeomeri, from their ischiadic artery. The fact 



' J. Y. MACKAY, ' The Development of the Branchial Arterial Arches in 

 Birds,' &c., Phil. Trans, clxxix. 1889, p. Ill ; J. F. VAN BEMMELEX, ' Die Visceral- 

 taschen u. Aortenbogen bei Keptilien u. Vogeln,' Zoul. Anz. ix. 1880, pp. 5'2(>, 

 543. 



