18 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
least amount of resistance to the water, while in the back- 
ward or effective stroke, a broad surface is presented. The 
bones of the shoulder-girdle and wing present exactly the 
same number and arrangement as obtains in the wing of a 
flying bird, with the single exception that the first phalanx 
of the first metacarpal is wanting, so that the Penguin has 
no thumb. The sternum has a deep keel, and thus 
resembles that of an ordinary flying bird, the muscles 
attached to it, however, being used for sub-aqueous instead 
of aerial flight. The scapula is very broad, and relatively 
to the size of the bird is larger than in any other group of 
birds. This is correlated in a very interesting way with 
the very great and quite exceptional development of the 
muscles which act on the shoulder joint, as it is chiefly 
from the shoulder that the movement of the paddle-lke 
wing is effected. On the other hand, there is atrophy of 
the muscles of the fore-arm and hand, consequent on the 
comparative fixity of the joints below the elbow, the move- 
ments of these joints taking on a somewhat rotatory 
screw-like character. 
The most striking point about all the bones of the 
fore-limb is their remarkably compressed flattened character, 
developed thus in harmony with the needs of the paddle- 
like wing. But from the developmental point of view, the 
most interesting bone the bird possesses is the tarso- 
metatarsal, the different segments of which, instead of 
being fused into one uniform bone, as is universal in all 
other known birds, show a definite tendency to a division 
into three parts by means of two deep longitudinal grooves, 
the upper end of each groove being perforated by a for- 
amen. | 
This fact alone points to a great antiquity for these 
birds, for it clearly indicates that the ancestral Penguin 
must have branched off from the primitive avian stem 
