PHTLOGENY OF THE PAL^OGKNATILffi AND NEOGNATH2E. 219 



The sternal plate of Casuarius is of great thickness and highly pneumatic. 

 Such specific differences as I have been able to find will appear in the " Key." 

 The sternum of Rhea resembles that of Dromceus and Casuarius in several respects, 

 but is yet very readily distinguishable from either. 



The coracoid groove, though shallow, is sharply defined. The outer and inner lips, 

 though but feebly developed, are subequal in length. The groove of the left side is 

 separated from its fellow of the right by a smooth-lipped and gently hollowed plate 

 of bone representing the anterior border of the sternum, equal in length to the groove 

 itself. The anterior lateral processes are large. 



The articular surfaces for the ribs are closely crowded, and follow immediately 

 behind the anterior lateral process. Thus the distance from behind the articular 

 surface for the last rib to the hinder border is more than twice the distance of the 

 space separating the coracoid grooves ; or, in other words, the articular surfaces of the 

 ribs occupy only one-sixth of the whole lateral sternal border, in Casuarius it occupies 

 about one-third. 



The body of the sternum is notched posteriorly and tapers from before backwards. 

 It presents a deep basin-like hollow dorsad and is produced ventrad into a long, 

 swollen, ridge-like prominence. The corpus sterni is much thinner than in Dromceus 

 or Casuarius. 



The articular surfaces for the sternal ribs are each more or less distinctly divided 

 into two, the outer being the larger. The pocket-like interarticular spaces (incisurse 

 intercostales) are of great depth. 



Large pneumatic apertures open below the base of the anterior lateral process. For 

 further remarks on the sternum of Rhea, see p. 224. 



In Struthio the sternum is roughly about as broad as long. It resembles that of 

 Dromceus in some respects and Rhea in others. The form and position of its coracoid 

 grooves resemble that of Dromceus. The ventral lip, however, is wider than the dorsal. 

 Both dorsal and ventral lips converge mesiad. The right and left grooves are separated 

 by a slight hollowing out of the anterior sternal border. The anterior lateral processes 

 are moderately well developed and project outward and forward. 



The articular surfaces for the sternal ribs are of great width and occupy nearly the 

 whole lateral border of the sternum, thus differing from both Dromceus and Rhea. 

 The pocket-like pneumatic incisurse intercostales are relatively less deep than in the 

 foregoing forms. The median portion of the bar serving for the articulation of the 

 ribs has lost its glenoid surface, which is preserved only at either end. In this point 

 Struthio and Rhea agree. 



There are large posterior lateral and in some sterna incipient median lateral processes. 

 The ventral prominence of the sternum lies behind the centre of the sternal plate, 

 and is broader and more flattened than in Rhea ; in which, by the way, it lies in the 

 middle of the sternal plate. 



