32 



THE VOYAGE OF H.M.S. CHALLENGER. 



this point the anterior border of the bone is almost straight. The posterior border of the 

 bone is sKghtly concave. Its inner surface is flat and uniformly smooth, while the outer 

 surface presents two well-defined grooves. Of these one extends along the whole length 

 of, and lies close to the anterior margin of the bone. It accommodates the tendon of the 

 extensor metacarpi radialis muscle. The second groove passes from behind obhquely 

 downwards and forwards, and is situated close to the lower end of the bone. It indicates 

 the course of the extensor proprius indicis muscle. The lower extremity of the radius 

 is provided with a surface of irregular form for articulation wdth the radial carpal bone. 

 The radius articulates with the ulna by means of two small articular facets, one at the 

 upper and the other at the lower end of the bone, in such a manner that it lies directly 

 in front of and parallel with the ulna. The movement permissible between the two bones 

 of the forearm is extremely limited, an arrangement which confers greater rigidity on 

 the wing as a whole, and increases its power as a flexible paddle. 



The radius presents precisely the same configuration in every species of Penguin. 

 The table shows the dimensions of the radius of the various species examined, in inches. 



TJie Carpal Bones. 



The carpal bones in the adult Penguin, as in other birds, are two in number. They 

 differ, however, in form and mode of articulation from those of other bii"ds, in accordance 

 with the altered form of the wing and its adaptation as a paddle to the peculiar require- 

 ments of the Penguins. 



The radial caijxtl bone is the smaller of the two, and consists of a small osseous 

 cube, interposed between the lower end of the radius and the upper extremity of the 



