OSSEOUS SYSTEM OF AVES. 



25 



middle line, and thence, according to the stimnlus of the growth 

 and pressnre of the pectoral muscles, extending, as a keel, into 

 the interspace. A separate ossification answering to the epi- 

 sternnm in Lizards and Crocodiles is not formed : but the body of 

 the sternum with the keel has a centre distinct from that of the 

 long bifurcate side-processes, exceptionally, in GallincB. 



In the Penguins, fig. 19, the sternum is long and narrow, with 

 a deep fissure,/*, on each side of the posterior border: the free 

 borders of the well-developed keel are straight, and meet at an 

 acute angle, which almost touches the furculum. There is a 

 short manubrium, e, behind which the coracoid grooves, h, meet. 



19 



sternum, scapular nrcli, and limb. Penguin {Aptenodijtes). 



In the Auks and Guillemots the sternum ' is very long and 

 narrow ; the lower border of the keel is convex, the front one 

 concave ; the manubrium is short and wedge-shaped ; the sternum 

 is entire in Alca imjiennis ; but has a narrow notch on each side 

 the posterior border in Alca tarda, to which the pectoral ridge ex- 

 tends from the costal border ; the notches are converted into fora- 

 mina in Uria and PJialeris lyygmcEa, The Loons ( Cohjmhus) have 

 a similar two-notched sternum, but with larger costal borders. 

 In the Grebes (^Podiceps), the sternum is broader ; and there is a 

 median notch between the two lateral posterior ones. In the 



