58 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
notched, or prolonged into one or two long processes. In the ostriches 
the ventral surface is smooth and this was formerly used as a character 
separating these birds as a group of ratites, in contrast to all other 
birds (carinate) which either use their wings in flight or in swimming 
(penguins) and in which there is a necessity for strong wing muscles. 
For the attachment of these the ventral surface of the sternum is de- 
‘veloped into a strong projecting keel (carina). It is to be noted that 
a similar keel is developed in the bats and pterodactyls. 
Fic. 56.—Sternum, etc., of Fic. 57.—Sternum of guinea 
Iguana tuberculata, after Blan- pig. sr, sternal rib; st, sterne- 
chard. c, coracoid; cl, clavicle; bre; ur, vertebral rib, x, xiphi- 
e, episternum; #, humerus; pc, sternum. j 
procoracoid; x, xiphisternum. 
In the mammals the number of ribs connected with the sternum 
is greater than in the lower classes. The sternebre may remain dis- 
tinct throughout life (fig. 57) or, as in man, they may fuse into fewer 
elements, the xiphoid process being unconnected with the ribs. In the 
edentates and rodents elements known as ossa suprasternalia and pro- 
sternum occasionally occur in front of the sternum, the significance of 
which is unknown. It is possible that traces of them persist in the 
higher orders, even in man (fig. 55). 
