ul INTERCLAVICLE 38 
— os — —_—— Se 
piece, apparently comparable to a xiphoid process, has been 
detected. 
Fic. 20.—Sternum of Rudolphi’s Whale (Ba/- Fia. 21.—Sternum of a young 
aenoptera borealis), showing its relation to Dugong (Halicore indicus). 
the inferior extremities of the first pair of x#. From a specimen in 
ribs. x 5. (From Flower’s Osteology. ) the Leyden Museum. ps, 
Presternum ; xs, xiphister- 
num. (From Flower’s Oste- 
ology). 
From the instances which have been described, as well as 
from the mode of development of the sternum and from the 
number of free ribs, 7.e. ribs which are not attached to it, it 
would seem that the sternum has undergone a considerable reduc- 
tion in its size. This reduction may be possibly accounted for 
by the need for respiratory activity, which is clearly increased by 
a less-marked fixity of the walls of the thoracic cavity. In the 
case of the Whales one can hardly help coming to that conclusion. 
The arrangement in the Monotremata does not, however, point in 
the same direction; for these animals are precisely like the 
higher Mammalia in the reduction of the sternum and of the 
number of ribs-which reach it. 
The Episternum.—The Mammalia are as a rule to be dis- 
tinguished from lower Vertebrates by the absence of an episternum, 
or interclavicle as it is also called. In the Monotremata, however, 
there is a large T-shaped bone which does not overlie the ster- 
num as in reptiles, but is anterior to it. ‘The relations of this 
bone to the clavicles seem to leave no doubt that it is the 
equivalent of the Lacertilian interclavicle or episternum. The 
Monotremata are not, however, the only mammals in which this 
structure is to be seen. The Mole in the embryonic condition is 
VOLE x D 
