STRUCTURAL ADAPTATIONS 383 



changes in the skeleton as striking as those which have devel- 

 oped in response to the needs of flight. The main features in 

 these changes are the elongation and narrowing of the pelvis, 

 and the backward shifting of the hind-limb. 



The Ducks, Auks and Guillemots {AlcidcB), Cormorants, and 

 Grebes and Divers afford the most favourable material for the 

 study of the various phases which are to be met with in this 

 connection. 



The Cormorants, for example, belong to a group including 

 some very diverse forms, all of which swim, though only the 

 Cormorants and Darters procure their food by chasing their 

 prey entirely beneath the surface of the water. 



In the Boatswain-birds and Frigate-birds {Fregata) the 

 pelvic girdle is moderately broad and has evidently undergone 

 but little modification. It is characterised by the fact that the 

 preacetabular ilium — that portion of the ilium lying in front of 

 the acetabulum or socket for the thigh-bone — is longer than the 

 post-acetabular moiety ; and that the two innominate bones 

 which make up the girdle are widely separated by long trans- 

 verse processes developed by the vertebrae. In the Cormorants 

 the post-acetabular ilium is by far the longest, while the trans- 

 verse processes are extremely reduced, so that the innominate 

 bones come to approach one another in the middle line; thus 

 the width across the pelvis is greatly reduced. 



With this elongation of the innominate and the lengthening 

 of its posterior portion there has gone on, simultaneously, a 

 reduction of what is known as the dorsal plane, that is, of the 

 flattened tabular upper surface of the post-acetabular region, 

 which reaches its climax in the Grebes and Divers. 



In these forms the innominate bone has assumed the form 

 of a long blade, divided by the socket for the femur into a short 

 anterior and a long posterior region. This blade is closely 

 applied to the side of a long series of fused vertebrae forming 

 the " synsacrum " (111. 42, p. 387). This has also undergone pro- 

 found modification, inasmuch as not only have the transverse 

 processes of the ribs completely disappeared but the centra of the 

 vertebrae have undergone an extensive side-to-side compression. 

 In the complete pelvis, therefore, little more than a knife-edge 

 is presented in a dorsal view. From the under surface it will 

 be seen that the separate cavities for the reception of the 

 kidneys have been entirely suppressed. 



