432 AVES. 



which are clearly lumbar (presacral) ; then follows the true 

 sacrum consisting of two vertebrae with their sacral ribs ; finally 

 comes the postsacral portion of the compound sacrum, which 

 consists of from three to seven of the anterior caudal vertebrae. 

 The short caudal region, which succeeds the postsacral, consists, 

 as a rule, of from seven to eight moveable vertebrae, of which the 

 last is represented by a vertical, laterally compressed plate, the 

 pygostyle, which supj)orts the tail-feathers and the uroj^ygial 

 gland. This deep ploughshare-shaped terminal bone is com- 

 posed of from four to six fused vertebrae, so that the reduction 

 of the number of caudal vertebrae, as compared with the num- 

 ber in the tail of Archaeopteryx, is not very great. 



The moveable vertebrae are separated by synovial cavities, 

 each of which is divided into two by a plate of fibro-cartilage, 

 called the meniscus. The meniscus is perforated by an aperture 

 which transmits a fibrous cord which is a remnant of the noto- 

 chord. 



The sternum (St) is a broad bone which covers not only the 

 thorax but a great part of the abdomen and bears a projecting 

 keel -like crest which serves for the attachment of the great 

 pectoral muscles (Carinatac). The keel is reduced or obsolete 

 onty when the power of flight is feeble or absent {Ratitae, Strin- 

 gops). The postero-lateral part of the sternum may be fenes- 

 trated, giving rise to vacuities or notches in the bone. 



The spina sternalis or rostrum is the anterior continuation of tlie sternum 

 between- the articulation of the coracoids. In some birds it consists of a 

 dorsal {spina interna) and ventral {spina externa) portion. 



The pectoral girdle consists of a scapula, a coracoid, and a 

 clavicle. The scapula {8c) is a sabre-shaped bone lying along 

 the dorsal side of the thoracic framework. Its anterior end is 

 expanded and firmly united by ligament (usually not ankylosed) 

 with the coracoid {Co), which is directed ventralwards and 

 articulates with a groove on the anterolateral edge of the ster- 

 num. In the Ratitae the long axis of the scapula is nearly in the 

 same straight line as that of the coracoid, but in most birds it 

 forms an angle less than a right angle with the coracoid (Fig. 239). 

 Both bones contribute about equally to the glenoid cavity. 

 The glenoidal end of the scapula is produced into an acromial 

 process, to which the clavicle is attached. The clavicle is also 



