56 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the rib behind and strengthening the thorax. In the chelonia the ribs 
are confined to the dorsal side of the body and are fused to the costal 
plates (dermal skeleton) to form the carapace. Single- and double- 
headed ribs often occur in the same individual of various groups, and 
in the mammals the capitular head, instead of articulating with a 
distinct parapophysis, may rest in a socket formed by two successive 
vertebre. 
Fic. 54.—Sacral vertebre, ribs and pelvis of Trionyx, obliquely from below. f, head and 
trochanter of femur; #, ilium; zs, ischium; ~, pubis; sv, sacral ribs; sv, sacral vertebra. 
The pelvis is never directly united to the sacrum, but sacral ribs 
intervene. These are distinct in the reptiles (fig. 54), but are fused to 
the transverse processes in other groups. 
THE STERNUM (BREASTBONE). 
The sternum includes the skeletal parts on the ventral side of the 
body, which are closely connected with the shoulder girdle and, except 
in the amphibia, with the ribs. The fact that it occurs only in verte- 
brates with legs (it is lacking in snakes and cecilians) shows that it has 
arisen in adaptation to terrestrial locomotion. In man it consists of 
three parts, a manubrium in front, a middle piece (gladiolus), and a 
xiphoid (ensiform) process behind, and these terms have been car- 
ried into other groups. 
In development the sternum arises in mammals by the formation of a longi- 
tudinal bar of cartilage in the linea alba on either side, ventral (medial) to the ends 
