120 STRUCTUEE AND CLASSIFICATION OF BIRDS 



degeneration, of floating lumbar ribs. Morphologically 

 equivalent with ribs are the processes firmly ankylosed to 

 the cervical vertebrae, which form a canal for the vertebral 

 artery ; these are, as a rule, absent from the atlas, but are 

 present on that vertebra in the Anseres, Opisthocomus, 1 

 Triponax Feddeni, Dromccus' 2 (fig. 66). Arcliceopteryx is 

 alone in possessing the abdominal ribs of the crocodiles and 

 other reptiles. 



The Shoulder Girdle. 3 The shoulder girdle of birds consists 

 of at any rate three separate elements the scapula above ; 

 the coracoid, articulating with the sternum ; and the clavicles, 

 generally united into a U-shaped piece. Of these the first 

 two are preformed in cartilage, the last in membrane. 



The scapula is a thinnish sword-shaped bone which 

 is attached by muscles to the ribs and to the vertebrae, 

 and lies in a direction, as a rule, nearly parallel to the 

 long axis of the body. The scapula does not show great 

 variability of form among birds ; the most considerable 

 variation is to be seen in the penguins, where the bone is, 

 comparatively speaking, of enormous width. A free supra- 

 scapula has been noted by PARKER in Opisthocomus. The 

 coracoids articulate on the one hand with the scapula, and 

 on the other with the sternum, where they are received into 

 grooves on its anterior margin. There are some variations 

 in the way in which these grooves are arranged : in some 

 birds the two coracoids at their insertion are not in contact 

 at all ; in others they are in contact ; and finally they may 

 overlap, as in reptiles. 



The coracoid has in many birds a procoracoid process, 

 which is believed to be the equivalent of the procoracoid of 

 reptiles. This is especially prominent in the ostrich, but is 

 present in a large number of other birds, though more 

 reduced in extent. But its large or small size is so capricious 



Stated by PAEKEE to be absent. 



MIVAET figures a canal on one side of the atlas of the ostrich. 



'' A. SABATIEE, Comparaison des Ceintiircs ct dcs Membres Ant&rieurs ct 

 Postcrlcurs dans la Seric dcs Verti'bres, Montpellier, 1880 ; PARKEB, quoted 

 below. See also LUHDEE in J. f. O. 1871, p. 321. 



