18 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



names have been derived. The Ratitae (fi'om ratis, a flat-bot- 

 tomed boat, in allusion to the flat sternum), which, as we have 

 seen, comprehend the old order Cursores, or the Ostriches and 

 their allies, are characterized by having no carina or median keel 

 to the sternum, which forms merely an oblong and slightly con- 

 vex plate of bone, presenting many points of resemblance to the 

 sternum of a reptile. The sternum of the Ratitse ossifies solely 

 from paired centres. 



In the Carinatse, on the other hand, the sternum (as in the 

 three figured specimens) always possesses a carina, and ossifies 

 from a median centre in that carina, as well as from lateral 

 paired centres.* (The term Carinatse is derived from carina, 

 a keeled vessel.) 



Very considerable variations in the form of the sternum oc- 

 cur among the CarinataB, which are in great part due to the 

 relative development of the notches and processes. In the 

 Tinamous {Dromceognathce), the notch between the middle and 

 internal xiphoid processes extends nearly to the summit of the 

 sternum, the internal xiphoid processes forming long and slender 

 rods, and the external xiphoid processes being undeveloped. 

 The whole sternum is excessively slender. In the Ducks, Auks, 

 many Waders, and diurnal birds of prey, the sternum, though 

 of ordinary breadth, is of enormous length, and extends down to 

 or even beyond the pelvis. In the Swifts and Humming-birds, 

 some Eagles and Petrels, and some other birds, there are no 

 notches or perforations in the sternum, and the sternum is hence 

 called entire. The sternum of the Frigate bird ( Tachypetes) repre- 

 sented in figure 2 of plate III, is " entire," and is noticeable 

 for its excessive breadth and shortness. The sternum of Phoe- 

 norhina (one of the Columbidge) represented in figure 1 of the 

 same plate, is an example of an average-shaped sternum, with 

 the notches fairly developed. The sternum of the fowl (fig. 3) 

 has the notches extending much higher up, and the processes 

 in consequence much more separated and distinct. In the 

 greater number of birds belonging to the old order Passeres, 

 the sternum is of average width and breadth, has large costal 

 processes, a well-developed rostrum, and only a single moderate- 

 sized notch inferiorly. 



Shoulder-girdle. — We now come to the consideration of the 

 bones forming the shoulder-girdle, which in birds consists of 

 three bones — the scapula'\ (pi I, sc), the coracoidX (pi. I, III, 



* It is possible that tte peculiar parrot, Strigops, may be an exception to this rule, 

 f Scapula, the shoulder blade. 



J Coracoid, the name coracoid, (from Kopai, a raven, and ticoi like), was ori- 

 ginally applied to a process of the human scapula, which was supposed to resemble 

 a raven's beak. 



