65G THE NATURAL HISTORT REYIEW. 



able from ordinary cartilage, tliougli its pliysical properties are some- 

 what different. The elastic fibrous network found in the cord, 

 confers upon it a high degree of elasticity— and it appears that it 

 admits of and follows the extension of the cartilaginous nucleus, 

 again enabling it to assume its original form. Tame rabbits pre- 

 sent, in the construction of theii- episternals, essentially similar cha- 

 racters, only that the matrix of the internal cartilaginous core is 

 fibrous, and the axial cartilage of the so-called clavicular ligament is 

 composed of fibro- cartilage. If we now compare the construction we 

 have already described in other mammals, with that found in rabbits, 

 there can be little question that a fundamentally similar arrange- 

 ments of parts exists, modified in the latter case no doubt to some 

 extent, but still showing, that even with apparently rudimentary 

 clavicles, episternal structures are to be found. It even appears 

 that the lateral parts of the typical episternum diminish in length 

 more gradually than the clavicles themselves. The conditions above 

 described are such that the episternals, whether consisting of pure 

 cartilage or partially ossified, present a structure rendering them 

 immediately recognizable as integral portions of the skeleton. Tliese 

 conditions too, certainly include the more complete forms that 

 stand in close connection with those well known to exist in the 

 Armadilloes and Monotremes. If we now proceed from the consi- 

 deration of those structural characters under which the episternals 

 appear first in the Hamster, and then in the Hedgehog, and convince 

 ourselves of the connection of these formations with the analogous 

 ones found in the Mouse, DidelpJiys and Bast/ pus, it will not be 

 difficult for us to recognize them, and to comprehend their mor- 

 phological significance under less characteristic forms. 



If we now examine other mammals provided with clavicles, some 

 of the following peculiarities may be found in the episternal appa- 

 ratus. In the Mole (fig. 8), the broad sjjernal extremity of the very 



Fig. 8. 



Stenio-clavicular articulation of the Mole, as seen on perpendicular section. 

 S(, sternum; Up, episternum; ?«, median piece of episternum. 



