by synovial articulations with the segment before and behind, and has at either end an upper and lower hollowed surface, which, with the corresponding surfaces on the contiguous segment, form articulating facets for the double-headed sternal ribs. This mode of articulation curiously resembles that at the vertebral end of the rib. The xiphisternum is rather long and simple.

In the small Tree Anteater (Cycloturus didactylus) the presternum is very broad and trilobate, sending out lateral expansions behind the attachment of the clavicles to meet

FIG. 43. Side view of three mesosternal segments of a young Anteater (Myrmecophaga tamandua), showing the mode of articulation of the sternal rib (sr), copied from Parker's figure. mst the upper or inner surface of the mesosternal segment; sy the synovial articulation between the segments.

the first pair of ribs. The hinder, narrow part of the manubrium is segmented off from the larger anterior part, and resembles a mesosternal segment; but it is in front of the attachment of the second pair of ribs. The true mesosternal segments are six in number, of nearly equal width, high, rounded above, and compressed below, with a synovial cavity between each. The sternal ribs are articulated by a single oval condyle. The xiphisternum is long, stout, and styliform.

In the Armadillos (Dasypodide) the presternum is broad, and in Priodon gigas (Fig. 45, p. 114) strongly keeled. The mesosternal segments, four to six in number, are broad above, but very narrow below. They are connected by synovial joints to each other, and to the strongly ossified sternal ribs, which have broad, sub-bifid heads. The xiphisternum expands posteriorly into a wide cartilaginous flap.

In the Sloths the sternum is long and narrow. The Threetoed species (Bradypus) have a rather broad presternum, but with no prolongation in front of the attachment of the first rib. This is followed by eight small mesosternal segments, and a very small rounded xiphisternum. In the Two-toed Sloths (Cholapus) the presternum is narrow, slightly keeled, and forms a considerable projection in front of the attachment of the first rib. The mesosternum has twelve segments, and the xiphisternum is rudimentary or absent.

In the MARSUPIALIA the sternum presents no especial aberrant characteristics. The presternum is rather broad at the point of attachment of the first pair of ribs. Its anterior extremity often does not ossify. There are usually four quite distinct, elongated segments to the mesosternum, connected to one another by fibrous tissue, and sometimes completed at each end by epiphyses. The xiphisternum has an elongated, narrow, ossified portion, and terminates in a laterally expanded cartilage, which may contain one or two endosteal bony patches.

In the MONOTREMATA the Ornithorhynchus (Fig. 44) has a broad presternum (ps), with a small partially-ossified pro-osteon (po) in front of it; three keeled mesosternal segments (ms), which commence to ossify in pairs, and no xiphisternum. The Echidna agrees in all important respects, but it has an ossified xiphisternum, which in E. Bruijni consists of three metameric portions.

The T-shaped bone, interclavicle or episternum (ic) in front of the presternum, which connects it with the clavicle and is often completely fused with it, appears to have

FIG. 44.-Sternum and adjacent parts of the skeleton of a young Ornithorhynchus (O. anatinus). c clavicle; ic interclavicle; po pro-osteon (a part of the true sternum); ps presternum; ms mesosternum; srsternal ribs; ir intermediate ribs ; vr vertebral ribs.

no homologue among the other Mammalia, and belongs more properly to the shoulder-girdle than to the sternal apparatus.

CHAPTER VIII.

THE RIBS.

THE ribs form a series of long, narrow, and more or less flattened bones, extending laterally from the sides of the vertebral column, curving downwards towards the median line of the body below, and mostly joining the sides of the sternum.

Free ribs are normally only attached to the thoracic vertebræ, although, as before shown, certain parts, which may be serially homologous with ribs, are found in other regions of the vertebral column; but in such cases they become, almost always, ankylosed with their corresponding vertebræ. In the thoracic region, the ribs are never normally ankylosed with the vertebræ, but are articulated to them by synovial joints, which permit a certain, though limited, amount of motion.

As a general rule, the first thoracic rib joins the presternum or manubrium; sometimes, as in the Whalebone Whales, this is the only rib united to the sternum; but usually a larger number are so connected, while the more posterior are either attached by their extremities to the edges of the ribs in front of them, and thus indirectly join the sternum, or else they are quite free below, meeting no

part of the skeleton. These differences have given rise to the division into true ribs and false ribs (by no means good expressions), signifying those that join the sternum directly and those that do not; and of the latter, those that are free below are called floating ribs.

Each primary piece of cartilage, out of which one of the half hoops or ribs is developed, is, moreover, divided transversely into two portions, which assume different characters, as they usually undergo a different mode of ossification, and remain more or less distinguishable from each other during life. The portion nearest the vertebral column is called the vertebral rib. This is the larger segment, and becomes firmly ossified at an early period by ectostosis;1 it is the bone commonly spoken of as a "rib."

The portion towards the sternal extremity or sternal rib is usually imperfectly ossified, and always at first (except in Monotremes) by endostosis.2 Sometimes it remains permanently in a cartilaginous state; but, on the other hand, in some cases (as in Armadillos and Sloths) it becomes as firmly ossified as the vertebral ribs.

The vertebral ribs are variously connected with the sternal : by continuous cartilage, by intercalation of fibrous tracts, or by synovial joints.

Occasionally, in the Mammalia, an "intermediate " portion of the rib is segmented off, as in Reptiles; this is best developed in the Monotremata (Fig. 44, ir), where, however, it is only partially ossified by endostosis. other instances in which it occurs (e.g., Odontoceti) it is quite rudimentary.

In all

1 The bony deposit commencing at the surface, and advancing inwards, as in ordinary long bones.

2 The bony deposit being irregularly scattered throughout the cartilage, often beginning near its central part.