_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. The posterior ribs, however, do not directly articulate with that bone, but are either attached by their extremities to the edges of each rib in front of them, and thus only 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” 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. The portion of each rib nearest the vertebral column and that nearest the sternum differ in their characters, the latter being usually but imperfectly ossified, or remaining permanently cartilaginous. These are called “costal cartilages,” or when ossified “sternal ribs.” [Illustration: FIG. 14.—Sternum and strongly ossified sternal ribs of Great Armadillo (_Priodon gigas_). _ps_, Presternum; _xs_, xiphisternum.] In the anterior part of the thorax the vertebral extremity of each rib is divided into two parts, “head” or “capitulum,” and “tubercle”; the former is attached to the side of the body of the vertebra, the latter to its transverse process; the former attachment corresponds to the interspace between the vertebræ, the head of the rib commonly articulating partly with the hinder edge of the body of the vertebra antecedent to that which bears its tubercle. Hence the body of the last cervical vertebra usually supports part of the head of the first rib. In the posterior part of the series the capitular and tubercular attachments commonly coalesce, and the rib is attached solely to its corresponding vertebra. The number of pairs of ribs is of course the same as that of the thoracic vertebræ. The circumstance that in some of the Anomodont reptiles and Labyrinthodonts the capitula of the ribs articulate with the intercentral elements of the vertebral column has suggested, as in the instance of the chevron bones, that the intercentral capitular articulation of the ribs of mammals is a feature directly inherited from those extinct types by the gradual disappearance of the intercentra. _Appendicular Skeleton._—The appendicular portion of the framework consists, when completely developed, of two pairs of limbs, anterior and posterior (Fig. 15).