OSSEOUS SYSTEM OF AVES. 23 



the costal borders are co-extended therewith, fig. 56, p. 68, vol. i. 

 In most Gallince the lateral margins of the sternum are deeply- 

 concave : in the Guan {Penelope) almost angularly incised, with 

 the costal border on the anterior slope. In the Tinamou, fig. 14, 

 B, the long marghi beyond the short costal border is convex : in 

 many Waders (Platalea, Phoenicopterus, fig. 14, c) and Swimmers 

 (Procellaria, Diomedcea) the lateral borders are straight and paral- 

 lel, or nearly so: in Rhea, Casuarius, Dromaius, Notorms, fig. 16, 

 they converge to the hind border : in most birds the lateral borders 

 are moderately concave and diverge, figs. 15 and 20. The costal 

 border is thickened, and divided by the transverse articular ridges 

 for the hasmapophyses into hollows, which usually show pneumatic 

 foramina. The modifications of the posterior border will be noticed 

 in connection w^ith the sternal characteristics of orders, or other 

 groups, of Birds. 



The part of the sternum bearing the most direct relation to the 

 force with which the pectoral limbs are worked is the 'keel,' 

 figs. 18 and 19, s. In order to afford origin to the accumulated 

 fasciculi of the pectoral muscles, which otherwise would become 

 blended together over the middle of the sternum, this osseous crest 

 is extended downward, analogous to the cranial crest which inter- 

 venes to the temporal muscles in the carnivorous mammalia ; and 

 which, in like manner, indicates the power of the bite. 



The keel varies in depth, length, contour of the front and lower 

 borders, and degree of production, freedom, or otherwise of the 

 angle between those borders. The keel is long and deep in the 

 wino;less Auk and Penouin, relatins; to the mass of muscle 

 working the fore limbs as fins in.these excellent and habitual divers : 

 in the Penguin both the free borders are straight, and meet at 

 rather an acute angle, fig. 19. The keel is deep, descending ante- 

 riorly far below the furculum in most GallincB : it is remote from 

 the furculum in Limosa, Ibis, Scolopax ; but touches it in many 

 other Grallatores {Otis, Psophia, Ciconia). It coalesces with the 

 furculum in Griis Virgo and Grus Antigone ; in the stilted Vul- 

 ture {Gypogeranus)\ in the Frigate-bird {Tacliypetes)', also in 

 the Pelican {Onocrotalus), Grannet (^Sula), and in old Cormorants 

 ( Carbo), the fore part of the keel being much produced in these 

 Totipalmates. The keel is thick in the few birds in wdiich a 

 fold of the windpipe penetrates it ; the anterior border being 

 excavated to admit the fold. In the larger Rcq^tores the front 

 border of the sternum is rather thick and subcarinate. The outer 

 surface of the sternum shows in many birds a ' carinal ' ridge, a 

 ' subcostal ' ridge and a ' pectoral ' ridge, the latter defining the 



