32 THE STORY OF THE BIRDS 



Ratite Birds, the Frigate Bird, and the now 

 extinct Dodo the coracoids and the shoulder- 

 blades are fused together. Two very distinct 

 types of sternum exist. Of these by far the 

 most generally prevailing is that in which the 

 ventral surface is provided with a ridge or keel 

 (carina). The other type of sternum is that in 

 which the keel is absent, more resembling a flat- 

 bottomed boat (ratis). This latter is found in a 

 few archaic types only, such as the Ostrich, Rhea, 

 Cassowary, Emu, and Kiwi. Birds with a keel 

 to the sternum are therefore termed Carinate 

 Birds, whilst those in which this ridge is absent 

 are called Ratita Birds, these groups forming 

 the two divisions into which all living birds are 

 separated by systematists. Before leaving the 

 sternum we have to allude to the various pro- 

 cesses on the sides behind the articulation of 

 the ribs. The most noticeable of these are 

 situated near the posterior portion, and consist 

 of outgrowths (although connected by mem- 

 branaceous tissue), giving the margin when 

 macerated a more or less deeply notched ap- 

 pearance, or when the extremities are joined 

 or enclosed by bone or cartilage they form 

 what are technically termed fenestrac. In some 

 birds two of these notches on each side of 



