XII 



PHYLUM CHORDATA 



465 



and the crocodile, but formed of bone and ankylosed with 

 the rib. 



Following upon the fourth thoracic are about twelve 

 vertebrae all fused into a single mass (Fig. 276, s. scr), and 

 giving attachment laterally to the 

 immense pelvic girdle. The 

 whole of this group of vertebras 

 has, therefore, the function of a 

 sacrum, differing from that of a 

 reptile in the large number of 

 vertebrae composing it. The 

 first of them bears a pair of free 

 ribs, and is, therefore, the fifth 

 or last thoracic (th. v. 5). The 

 next five or six have no free 

 ribs, and may be looked upon 

 as lumbar (Fig. 278, I 1 — s 3 ). 

 Next come two sacral vertebras 

 (c l ) homologous with those of 

 the lizard. The remaining five 

 vertebras of the pelvic region are 

 caudal. Thus the mass of ver- 

 tebrae supporting the pelvic girdle 

 in the pigeon is a compound 

 sacrum, or syn-sacrum, formed by the fusion of the posterior 

 thoracic, all the lumbar and sacral, and the anterior caudal 

 vertebras. 



The syn-sacrum k followed by six free caudals and the 

 vertebral column ends posteriorly in an upturned, compressed 

 bone, the pygostyle or ploughshare-bone (Fig. 276, pyg. si), 

 formed by the fusion of four or more of the hindermost 

 caudal vertebrae. 



The sternum (Fig. 276, si) is one of the most character- 



2H 



Fig. 278. — Columba livia. Sac- 

 rum of a nestling (about 

 fourteen days old), ventral 

 aspect, f 1 , centrum of first 

 sacral vertebra; c 1 , centrum of 

 fifth caudal; c. r, first sacral 

 rib; Z 1 , centrum of first lumbar; 

 P, of thirdlumbar; s 1 , of fourth 

 lumbar; J 3 , of sixth lumbar; 

 tr. p, transverse process of 

 first lumbar; tr. p' , of fifth 

 lumbar; tr p" , of first sacral. 

 (From Parker's Zootomy.) 



