SKELETON OF THE MUSK-BUCE. 403 



diving, since a precisely similar modification obtains in the Grebes 

 and Divers. 



The pelvis of Mergus, among the Ducks, approaches nearest, in 

 its general conformation, to t\\^X, oi Bizmra. In so far as the re- 

 duction in the width of the postacetabular ilium is concerned, 

 indeed, the two genera are practically identical. In both, this 

 portion of the innominate is represented only by a narrow blade 

 of bone twisted so as to lie almost vertically. The peculiarly 

 narrow pelvis of Biziura is brought about by the reduction of 

 the transverse processes of the synsacral vertebrae. Between the 

 pre-ilia, indeed, even the neural spines have become reduced (in 

 thickness), so that innominates of this region meet in the middle 

 line to form an almost knife-like edge. In the region between 

 the antitrochanters the pelvis is normally at its broadest. In 

 Mergus the innominates are separated one from another for half 

 an inch by the transverse processes of the sacral and neighbour- 

 ing vertebrae, but in Biziura they are divided only by a slightly 

 swollen neural crest. As this is traced backwards the shelf 

 formed by the transverse processes gradually appears, but at its 

 widest the ilia are not separated by more than "4 of an inch. 



Since this extremely narrow type of pelvis occurs both in 

 marine and freshwater species, it would seem either that Eider- 

 Ducks and Scoters, now almost exclusively marine, have only 

 recently become confined to this habitat ; or that for some 

 unsuspected reason they have escaped the transforming effects of 

 the environment, so obvious ia such homoplasts as the Colymbi 

 and Alcidse, for example. 



The ilio-ischiadic foramen is relatively larger in Biziura than 

 in any other of the diving ducks. In the Fuligulioge it is 

 divided into two by a broad median bar. 



The pubes of Biziura have very long free ends curving 

 abruptly inwards towards the middle line, so as to very nearly 

 touch one another. In Mergus^ (Edemia, and Somateria the free 

 ends of the pubes are yet longer, but are directed downwards 

 and slightly backwards, curving inwards at the same time. The 

 pectineal process is well-developed. 



The Pectoeal Limb. 



In Biziura the humerus is slightly longer than the fore-arm, 

 and in this it agrees with Urismatiira, Somateria, (Edemia, 

 Mergus, and Fuligula, for example. The manus and fore-arm 



