ON THE EPISTEENAL APPARATUS OF MAMMALS. 547 



between the clavicle and episternum, tlie connection between 

 the two being however very firm and close. The transverse arms are 

 usually directed backwards towards the expanded basis of the epi- 

 sternum, so that the clavicles appear to be attached much nearer to 

 the first rib than is actually the case, whilst they possess a firm 

 support in the anterior parts of the bony sternum. That position of 

 the clavicles is most natural in which they abut upon the sternum, 

 nor can this relation be materially altered, except by the yielding of 

 some of the ligamentous bands retaining them in position, consequent 

 upon which the transverse processes may readily be drawn forward, 

 and their long axes made to fall in the same plane. Perhaps this 

 mode of union of the clavicles in the Monotremata together with the 

 backward direction of the transverse branches of the T-shaped 

 cartilage, has been the cause of the neglect of these parts of the 

 skeleton in animals w^hose anatomy has been otherwise so carefully 

 studied. But it may readily be shown that the relations of these struc- 

 tures are precisely the same both in the marsupials and in the mono- 

 tremes, though they are more highly developed in the latter. In the 

 former they are both smaller and remain persistently cartilaginous, 

 whence it follows that a less firm basis of support is offered for the 

 clavicles, leading to the bending downwards of the transverse processes 

 towards the sternum, by which means the firmer support of the side 

 of the median portion of the episternum is obtained. Now as there 

 is no joint nor marked line of differentiation between the osseous 

 anterior portion of the proper sternum and the cartilaginous piece 

 already mentioned, it would be possible to regard the whole structure 

 as properly belonging to the sternum. But we must take into con- 

 sideration that in other parts of the body, portions of the skeleton 

 frequently become connate, or intimately connected with one another, 

 and that the want of independency of the episternum may really 

 proceed from defective ossification.* For in many cases the whole 

 si^rnum is composed originally of undivided cartilage, the di\ ision 



* I must here remark that I have only had the opportunity of examining young 

 animals, so that it is quite possible that a point of ossification may subsequently 

 appear in the azygous central portion of the cartilaginous episternum. Others will 

 be able to decide this question, and will also determine the relations of the epister- 

 num in other genera of the marsupials, respecting which I can furnish no informa- 

 tion. In a dry preparation of Ifalma turns I found at the sternal end of each 

 of the clavicles what appeared to be a cartilaginous mass, through which the 

 connection with the sternum appeared to be effected. But I think it unsafe to draw 

 any conclusions fi-om such dry specimens, and only intend by thus mentioning them 

 to excite the attention of other observers. 



