INTERCLAVICLE 



33 



piece, apparently comparable to a xiphoid process, has been 

 detected. 



FIG. 20. Steruum of Rudolphi's Whale (Bal- 

 aenoptera borealis), showing its relation to 

 the inferior extremities of the first pair of 

 ribs. x Jy. (From Flower's Osteology. ) 



FIG. 21. Sternum of a young 

 Dugong (Halicore indicus). 

 x J. From a specimen in 

 the Leydeu Museum, ps, 

 Presternum ; xs, xiphister- 

 niim. (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, i.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 Monotrernata 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 



VOL. x D 



