concerned in the movements of flight. In many forms this part of the sternum is much broader than the pieces which follow; this is so with the Viscacha. In the Pig the precise reverse is seen, the manubrium being narrower than the rest of the sternal bonelets. It will be noticed, however, that in this and similar cases there are no clavicles. Ribs are attached between the successive pieces of the sternum. When the sternum is reduced, as it is in the Cetacea and in the Sirenia, it is the intermediate part of the series of bones which becomes abbreviated or vanishes. The Sperm Whale has only a manubrium sterni and a following piece belonging to the mesosternum. It is fair to say that the xiphoid process and the rest of the sternum have disappeared, since among the Toothed Whales a progressive shortening of the sternum can be seen. In the Whalebone Whales the sternum is still further reduced; the manubrium is alone left, and to it are attached but a single pair of ribs. In _Balaena_, however, a rudimentary {33} piece, apparently comparable to a xiphoid process, has been detected. [Illustration] [Illustration] FIG. 20.--Sternum of Rudolphi's Whale (_Balaenoptera borealis_), showing its relation to the inferior extremities of the first pair of ribs. x 1/10. (From Flower's _Osteology_.) FIG. 21.--Sternum of a young Dugong (_Halicore indicus_). x 1/4. From a specimen in the Leyden Museum, _ps_, Presternum; _xs_, xiphisternum. (From Flower's _Osteology_). 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 reduction 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. [Illustration] FIG. 22.--Shoulder girdle of Ornithorhynchus. _c_^1, _c_^2, _c_^3, First, second, third ribs; _cl_, clavicle; _e.c_, epicoracoid; _es_' and _es_", interclavicle (episternum); _m.c_, metacoracoid; _m.s_, manubrium sterni; _sc_, scapula; _st_, sternebra. (From Wiedersheim's _Structure of Man_.) [Illustration] FIG. 23.--Episternum of an embryo Mole. (After A. Goette.) _cl_, Clavicle; _es_', central portion of the episternum; _es_", lateral portion of the same; _r.c_, costal ribs; _st_, sternum. (The figure was constructed from two consecutive horizontal sections.) (From Wiedersheim's _Structure of Man_.)