38 THE VOYAGE OF H.M.S. CHALLENGER. 



another takes place simultaneously with the forward or backward movement of the 

 organ as a whole. 



A tendency to this flattening of the alar skeleton is met with in the Auks and 

 Guillemots, but in these birds it is never developed to the same extent as in the 

 Penguins. 



Lastly, in the Penguins the first or radial metacarpal bone is inseparably anchylosed 

 with the second, and this, combined with the absence of phalanges appertaining to the 

 former, renders the wing of the Penguin destitute of the free " pollex " met with in the 

 majority of birds. 



The Posterior Extremity. 



The Pelvic Bones. 



The pelvis as a whole (PI. VII. figs. 7 and 8) is very narrow and much elongated. The 

 pre-acetabular portion of the ilium is flattened from above downwards. Both its surfaces 

 are smooth and almost flat. The anterior liorder of the iliac bone reaches as far forwards 

 as the tubercle of the eighth vertebral rib. Its pre-acetal)ular portion narrows gradually 

 from the anterior extremity backwards nearly to the acetabulum, immediately in front of 

 which, however, it is slightly expanded. It articulates with the twelve lumbo-sacral 

 vertebrae already described. The post-acetabular portion of the iliac bone is flattened 

 from side to side. Its upper margin forms a thin sharp crest of bone, from which the 

 biceps femoris arises. The lower border of the post-acetabular portion of the ilium 

 is anchylosed with the ischium, and in part forms the upper boundary of the sciatic 

 foramen. The latter is of large size and oval in form. The acetabulum presents 

 in the macerated bone a wide perforation, which, as usual among birds, is filled up by 

 fibrous membrane. The upper border of the acetabulum is provided with a very prominent 

 " antitrochanter " for articulation with the great trochanter of the femm*. 



The ischium is broad and flat. Its posterior border presents a sharply-defined 

 V-shaped notch, which forms the posterior border of the pelvic bone. 



The pubis is almost straight, and does not converge posteriorly towards its fellow 

 of the opposite side. It is a narrow but stout bone, which, except at its anterior 

 extremity, is of equal thickness throughout. The obturator foramen corresponds to the 

 entire length of the pubic bone, the latter, except where it enters into the formation of 

 the acetabular cavity, not coalescing with the ischium at any point. The anterior portion 

 of the obturator foramen is rounded, and transmits the tendon of the obturator internus 

 muscle. The circumference of the posterior portion of the obturator foramen is formed 

 exclusively by the fibrous membrane which attaches nearly the whole length of 

 the pubic bone to the lower margin of the ischium. It ought, however, to be observed, 

 that although the ischium and pubis are nowhere anchylosed, except in front, they come 

 into very close relation with one another immediately behind the anterior dilated portion 



