144 SHOULDER-GIRDLE AND BREAST-BONE. 



we find no bony centres ; and the lateral halves are imperfectly united at the mid-line (see 

 Plate XVI, fig. 2, which is that of a Pheasant-chick, magnified six diameters). Five parts are 

 seen in front, namely, the two costal processes (c. p.) externally ; the rostrum (r.) at the mid- 

 line, this is notched in front ; and between these a deep notch, bounded, submesially, by the 

 coracoid lips and groove (cr. g.). 



The costal processes pass into the costal region, with its four condyles for the sternal ribs 

 (c. c.) ; behind the fourth condyle the large, lateral xiphisternum is seen, which bifurcates into 

 the " external " and the " intermediate xiphoid processes " (e. x. and i. x.) ; the interspace between 

 these is the " external xiphoid notch." Between the " intermediate xiphoid process " and the main 

 part of the Sternum there is a very long notch, with only a moderate tract intervening between it 

 and the anterior notch ; if these clefts were perfect, we should have a large separate " ento- 

 sternum " (e. s.), with the emarginate rostrum in front, the coracoid grooves antero-laterally, a 

 large keel further backwards, and behind the keel the " middle xiphoid process " (m. x.) ; this 

 process is double, and its halves are separated by the primordial fissure. I shall describe the 

 smaller clefts, which tend to cut up the great " ento-sternal " mass into successive segments, when 

 I come to the various types ; I must, however, speak beforehand of the osseous centres which 

 appear in this great, single, sternal plate, potentially so compound, and yet with no complete 

 cleavage ; and although ossified from several centres, yet soon becoming absolutely single, even 

 in that respect. For the Bird's Breast-bone is of a very high morphological type, but extremely 

 unlike that of the Mammal, whose Sternum, often very perfectly segmented, has so little lateral 

 and vertical extension. 



The fewest ossifications appear in the Struthionidae ; these, with the exception of the genus 

 Eliea, have but one symmetrical pair ; these centres being principally related to the costal margin, 

 may each be called " pleurosteon " (see Plate XVII, figs. 3 and 4) ; they exist in all the Lacertilia 

 and the Crocodiles. In all other Birds, known to me, there is a large azygous centre, which 

 arises in the crest of the Sternum (see Plate XV, fig. 17, 1. o.) ; this may be termed the 

 " lophosteon," it is nearly universal in the Bird-class, but has been observed in no other Verte- 

 brates. Behind each "pleurosteon" there is, in the Gallinacese (see Plate XVI, fig. 10, m. o.), 

 and a few other types, in the Crows (see Plate XV, fig. 18, m. o.), for instance, another bony 

 centre, the " metosteon ;" it ossifies the outer and intermediate xiphoid bars in the Gallinaceae j 

 this centre exists in the third degree of frequency. 



In the Hemipods (Turnix) there is an ovoidal patch of bone on each side the " lophosteon," 

 each piece being about half the size of that centre; these (see Plate XVI, figs. 13, 14, c. o.) may 

 each be called " coracosteon :" it is probable that these may exist in the Tinamous ; but they are 

 rare, evidently. 



The xiphoid end of the " ento-sternum " " middle xiphisternal process "is heart-shaped 

 in the Cariama (DicJiolophus] ; it is almost a separate segment, and has in its centre a small bone 

 (see Plate XIV, figs. 11 and 12, u. o.) ; this rare bone in the tail-end of the ento-sternum may be 

 called the " urosteon." In the genus RJtea, besides the pair of centres seen in the other typical 

 Ostriches, there is, on each side, an osseous centre in front of the first rib : it ossifies the costal 

 process, and, projecting forwards as a wing in front of the sternal ribs, may be called the " pro- 

 osteon." These are all the bony centres I have seen in the Sternum of the Bird ; the " pleu- 

 rosteon " appears on each side in the Reptile, and the " metosteon " in the feeble osseous deposit 

 in each xiphisternal horn of Stellio (Plate- XI, figs. 1 and 2, x. st.) whilst the "pro-osteon " is to 



