THE EPISTERNUM.--The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large [18]-shaped bone which does not overlie the sternum 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 {34} provided with pieces of bone which overlie the manubrium sterni and are attached to the clavicles, and are no doubt to be regarded as the same structure. Probably in many mammals the manubrium will be found to be partly made up of corresponding rudiments. In any case, vestiges of an episternum in the shape of two minute ossicles have been discovered in Man, lying in front of the manubrium. They have been termed ossa suprasternalia. In Man and in the Mole the paired nature of the episternum is clearly apparent. It has been suggested that this structure in its entirety belongs to the clavicles, just as the sternum belongs to the ribs; _i.e._ that it formed out of the approximated and fused ends of the clavicles. Dr. Mivart[19] figured a good many years since a pair of ossicles in _Mycetes_, lying in one case between the ends of the clavicles and the manubrium sterni, and in another example anterior to the ventral ends of the clavicles. Gegenbaur has figured a {35} pair of similar bones in the Hamster.[20] It is possible that these are to be referred to the same category. It has also been suggested that these supposed episternal rudiments are the vestiges of a pair of cervical ribs. [Illustration] FIG. 24.--Episternal vestiges in Man. _cl_, Clavicle, sawn through; _es_, "episternum" (sternoclavicular cartilage); _l_', interclavicular ligament; _l_", costoclavicular ligament; _m.s_, manubrium sterni; _o.s_, ossa suprasternalia; _r.c_, first rib; _st_, sternum. (From Wiedersheim's _Structure of Man_.)