BIRDS. 191 



the Mammalia. Save in the ordinary Osseous Fishes and the Batrachia, nowhere else can ectosteal 

 " centres " be so well studied as in the Sternum of the Struthioninae and Gallinse ; whilst there 

 is no mode of ossification so confused and hurried, as in the typical ornithic Sternum : the Plates 

 show this well, for the bony laminae are made to look as though they could be slipped off. 

 A section through the " pleurosteon " of the Rhea is shown in fig. 8 ; and through that of the 

 Emeu in fig. 9 ; both these are magnified six diameters ; they both show that the outer lamina has 

 caused the setting up of endostosis in the outermost cartilage-cells. The posterior third of 

 the Sternum of the Rhea is soft, even in the adult. 



In Apteryx australis the Shoulder-girdle is essentially like that of the Rhea, but the scapula 

 (Plate XVII, fig. 1, sc. 1 ) is rounded and decurved at the end, as in Picus viridis (Plate 14, fig. 21) ; 

 it is also more arcuate, but the lower half of the scapula is not much bent upon the coracoid (cr.). 

 These two bones, as shown in the half-grown bird (fig. 2, sc. cr.), are separately ossified, as usual ; 

 there is a small pras-coracoid projection (p. cr.), and the coracoid is pierced (fig. 2) : the coracoid 

 (cr.) is half the length and twice the breadth of the scapula ; and the epicoracoid region (e. cr.) 

 is more arcuate than in the Rhea. In the adult (fig. 1) the coracoid, which has become com- 

 pletely fused with the scapula, is very much enlarged by fibrous ossification. As in the Rhea, 

 there are no clavicles. The Sternum is more aborted than in any other known Bird, being 

 altogether as flat and as thin as in the Stellio Lizard ; but it is wholly ossified. The ento- 

 sternum (e. st.) is greatly cut away in front ; but the great notch is double, for there is a slight 

 pointed projection at the mid-line. The costal processes (c. p.) form two large, rounded ears ; 

 behind these the Sternum is narrowed, then the costal condyles swell out, and then comes 

 the flat out-bent external xiphoid (e. x.), which is slightly notched on the inside, tending to 

 make intermediate processes. The xiphoid notches are deep and rounded, and the middle 

 xiphoid plate (m. x.) has sinuous sides ; it is tongue-like, very thin, and extends several lines 

 behind the external bars. The formation of the coracoid grooves (cr. g.) shows well their 

 secondary nature as " outgrowths," as in the embryos of higher types ; for they are entirely 

 outside the rest of the Sternum; and the relation of the Shoulder-girdle to the Sternum is 

 altogether secondary, and forms no part of the true axial skeleton, as the Transcendentalists vainly 

 teach. 



1 Fig. 1 is reduced to three fourths; and fig. 2 is full size. 



