24 ANATOMY OF VERTEBRATES. 



origin of the ' pectoralis secundus.' The subcostal ridge ^'aries 

 in its distance from the costal border, being more remote in Aquila, 

 e. g. than in Uria : the pectoral ridge varies in position, direction, 

 and extent. In the Eao-le it reaches from the outer end of the 

 coracoid groove to the middle of the base of the keel. In the 

 Razor-bill (AIca tor da) it extends from the costal border to the 

 posterior sternal notch ; these differences relate to the form and 

 proportion of the pectoralis secundus. The ' manubrium ' forms 

 but a small portion of the sternum, and is often absent or rudi- 

 mentary : it may be compressed, spatulate, long and simple, or 

 bifurcate; the latter is its character in all Contores, fig. 15, e. 



The parts of the sternum of the Lizard, fig. 17, homologous 

 with parts of the sternum of the Bird are those forming the 

 ' coracoid groove,' ib. h, the ' costal border,' ib. c, c, and the 

 median bone, 59, passing forward to join the clavicles, ib. 58. 

 The broad flat bone, including the first two parts, exists in all 

 birds ; the third, or ' episternal ' part, is wanting as a distinct 

 element, but its positions and connections are repeated by the 

 exogenous keel and ' manubrium.' The episternum, moreover, is 

 not present in all Lizards : it is wanting in the Chameleons, e. g., 

 in which the sterninn partakes of the simplicity of that in the 

 Notornis, fig. 16, the Apteryx and Emeu. 



In the Apteryx the anterior border of the sternum between 

 the coracoid grooves is concave, and the posterior border has a 

 deep and Avide emargination on each side. In the Emeu the 

 coracoid grooves meet at the middle of the anterior border ; and 

 the sternum contracts posteriorly to an obtuse point. The ster- 

 num is rhomboid, also, in the Cassowary : it is broader in pro- 

 portion to its length, and subquadrate in the Ostrich. In 

 Notornis, fig. 16, the costal borders converge posteriorly, as in 

 Lizards, and the narrow breast-bone is continued as a ' xiphoid ' 

 part, gradually contracting to a blunt point. The depressions, 60, a, 

 for the pectoral muscles are separated by a narrow median tract, 

 expanding anteriorly, 59, and showing the beginning of the ' keel.' 

 In Brachypteryx^ the keel is rather more prominent : two obtuse 

 ridges diverge from its fore-part to the coracoid grooves, between 

 which the fore margin is deejjly concave, as in the Apteryx. 

 There is no distinct ossific centre for the keel in Brachypteryx, 

 any more than in its feebler rudiment in Notoimis. In all these 

 keel-less sternums ossification begins, as in the Ostrich^, by a 

 pair of centres expanding until they meet and coalesce in the 



' XLiv. p. 238, no. 128U. - lb. p. 264, no. 136G. 



