362 THE VOYAGE OP H.M.S. CHALLENGER. 



the organism is prevented by the great development of the subcutaneous fatty layer 

 which is found in every member of the group. The muscular system is further 

 characterised by the great strength of all, and the peculiar disposition of certain of the 

 extensor muscles of the vertebral column, more especially of the biventer cervicis, which 

 extends from the iliac bone to the skull, these peculiarities being associated with the 

 erect attitude of the Penguin when on land ; by the presence and arrangement of the 

 transverse cloacal muscle ; by the great strength of the muscles which act at the shoulder- 

 joint, that is upon the w T ing as a whole ; by the peculiar disposition of the brachialis 

 internus muscle ; and by the almost complete atrophy of the muscles which act upon the 

 fore-arm and hand, the last-mentioned peculiarities being associated with the alteration 

 which has taken place in the function of the w T ing. As regards the muscles of the leg, 

 the Penguins do not differ essentially from other Palmipedes. 



" The peculiarities of the arterial system are no less striking than are those of the 

 bony and muscular systems. They consist in («) the presence of two common carotid 

 arteries of equal size, symmetrically disposed on either side of the middle line ; (b) the 

 peculiar mode of distribution of the subclavian artery, which breaks up into an axillary 

 and brachial rete mirabile from which branches are derived for the supply of the fore-arm 

 and hand ; (c) in the absence of the sciatic artery as a direct branch of the abdominal 

 aorta, and its substitution by a branch of the crural artery. 



" The respiratory organs closely resemble those of other birds in their general arrange- 

 ment. In one important particular, however, they differ from all with the exception of 

 those of the Procellariidse. The presence of a tracheal septum, which more or less 

 completely divides the air-tube into two lateral chambers, is met with only in the 

 Spheniscidse and in the Procellariidse. This tracheal septum is usually but not constantly 

 present in the Spheniscidse. I found it in all the forms which I examined, with the 

 exceptions of Eudyptes chrysocome from Tristan da Cunha and Spheniscus minor. The 

 occasional absence of a tracheal septum, therefore, shows that this structure, per se, 

 cannot be considered as a constantly reliable anatomical character of this group any more 

 than of the Procellariidse. 



"The geographical distribution of the Spheniscidse is of interest. 1 ' The various 

 members of the group are entirely confined to the southern hemisphere, not one single 

 species of Penguin being found north of the equator. In the southern hemisphere, 

 however, their distribution is very extensive, reaching from the Galapagos Islands on 

 the equator southwards to the Antarctic Islands. Of the various species of Penguin 

 collected by the Expedition and referred to in the Report, Spheniscus demersus is confined 

 to the vicinity of the Cape of Good Hope, Splieniscus magellanicus to that of Cape Horn, 

 Spheniscus mendiculus to the coast of Chili, while Spheniscus minor inhabits the South 

 Pacific, in the neighbourhood of Australia and New Zealand. The genus Eudyptes 



1 Wallace, Geographical Distribution of Animals, vol. ii. p. 366, 1876. 



