lower Vertebrates, where there is a gradual transition between the ribs of the cervical and those of the dorsal region of the vertebral column; for that of the seventh ribs in Monotremes is smaller than those which precede it. [Illustration] [Illustration] FIG. 18.--Sternum and sternal ribs of the Common Mole (_Talpa europaea_), with the clavicles (_cl_) and humeri (_H_); _M_, manubrium sterni. Nat. size. (From Flower's _Osteology_.) FIG. 19.--Sternum of the Pig (_Sus scrofa_). x 1/4. _ms_, Mesosternum; _ps_, presternum; _xs_, xiphisternum. (From Flower's _Osteology_.) THE STERNUM.--All the Mammalia so far as is known possess a sternum. This is the bone, or series of bones (sternebrae), which lies upon the ventral surface of the chest, and to which the ribs are attached below. The development of the sternum has been shown to take place from the fusion of the ribs below into two lateral bands, one on each side; the approximation of these bands forms the single and unpaired sternum of most mammals. Very considerable traces, however, of the paired state of the sternal bones often exist; thus in the Sperm Whale the first piece of the sternum is divided into two by a longitudinal division, and the second piece is longitudinally grooved. The development of the sternum out of the fused ends of ribs is shown in a more complete condition in some species of _Manis_ than in many other mammals. Thus in _M. tricuspis_ the last ribs of those which are attached to the sternum are completely fused together into a single piece on each side.[17] As a general rule the last ribs which come into relation with the sternum do so only in an imperfect way, being simply firmly attached at their sides to, but not fused with, the last ribs which are definitely articulated with the sternum. Contrary to what is found in lower {32} Vertebrates, the sternum of the Mammalia consists of a series of pieces, as many as eight or nine or even sixteen in _Choloepus_, of which the first is called the manubrium sterni, and the last the ensiform cartilage, xiphisternum, or xiphoid process. The latter often remains largely cartilaginous throughout life; in fact this is generally but not universally the case with that part of the breastbone. The most extraordinary modification of the xiphoid process is seen in the African species of the genus _Manis_, where it diverges into two long cartilages, which run back to the pelvis and then, curving round, run forwards and fuse together in the middle line anteriorly. These processes serve for the attachment of certain tongue-muscles. They were looked upon by Professor Parker as the equivalents of the "abdominal ribs" of reptiles elsewhere non-existent among mammals. This view is not, however, usually held. The manubrium sterni is often keeled in the middle line below; this is so with the Bats, which thus approach the birds, and probably for the same reason, i.e. the need of an enlarged origin for the pectoral muscle, which is