(_m_) The _ribs_ (fig. 8) are very diverse in structure and in their mode of articulation (541) with the vertebral column. The characters of the ribs and vertebræ constitute the best means of classification of these animals so far discovered. In the Branchiosauria the ribs are always straight, heavy, and short, and articulate intravertebrally upon a large and strong transverse process. They occur throughout the vertebral column. There is a single pair of sacral ribs which are not to be clearly distinguished from the pre-sacral and post-sacral series. The cervical and caudal ribs are shorter than the dorsal series. The branchiosaurian rib is composed almost entirely of perichondral ossification. It presents the same condition as does the humerus of the cow embryo of 2 to 3 inches in length. The ribs of the branchiosaurs are identical in every way with the ribs of modern salamanders and form one of the strong arguments in favor of the relationship of the Branchiosauria to the Caudata. Among the Microsauria the ribs are always long, slender, curved, and intercentral. They may be either single or double headed, but usually the former. They resemble in their characters the ribs of some of the early reptiles and an attempt has been made to relate the Microsauria (469) to the primitive reptiles on this basis. The ribs of the other groups are still unknown. Indeed, representatives of the Temnospondylia and the Stereospondylia are very scanty in the American Coal Measures. One large rib (plate 22, fig. 4) may represent a labyrinthodont, but nothing is known of the species to which the rib belonged.