290 VEKTEBEATE ANIMALS. 



CHAPTEE XXIX. 

 DIVISIONS OF BIRDS. 



Birds may be variously divided, but for our present purpose it is 

 most convenient to regard them as divided into the following three 

 great primary divisions or sub-classes : — 



1. Ratit^. — This division comprises the Running Birds, which 

 cannot fly, such as the Ostriches, Emeus, and Cassowaries. It is 

 characterised by the fact that the sternum has no median ridge or 

 keel for the attachment of the great pectoral muscles. The sternum 

 i.s therefore raft-like (Lat. rates, a raft), hence the name given to 

 this section. The barbs of the feathers are separate. 



2. Carinat^. — This comprises all living Birds except the preced- 

 ing, and is characterised by the fact that the sternum is furnished 

 with a prominent median ridge or keel {carina) ; hence the name 

 applied to the division. Under this head are included six smaller 

 divisions or orders. 



3. Odontornithes. — This sub - class comprises three orders of 

 fossil Birds, the essential characteristic of which is that they pos- 

 sessed true teeth, sunk in distinct sockets or grooves in the jaws. 



SUB-CLASS I. RATIT^. 

 Order Cursores. 



The Running or Cursorial Birds, comprising the Ostrich, Casso- 

 wary, Emeu, Rhea, and Apteryx, are characterised by the rudi- 

 mentary condition of the wings, which are useless as organs of flight, 

 and by the compensating length and strength of the legs. In ac- 

 cordance with this condition of the limbs, the bones have few air- 

 cells, and the breast-bone is destitute of the prominent ridge or keel 

 to which the great muscles of the wings are attached (fig. 206, A). 

 The two sides of the pelvis are united together below in the Ostrich, 

 and in al! the pelvic ai-ch has great strength and stability. The legs 



