In connexion with the elaboration of the chain of auditory ossicles it is very usual for mammals to possess a thin inflated bone, sometimes partly or entirely formed out of the tympanic bone, and known as the tympanic bulla. Whether this structure is thin and inflated or thick and depressed in form it is characteristic of the mammals, and does not occur below them in the series. But it is not present in all mammals. It is absent, for example, in the Monotremes. When it is present it is sometimes formed from other bones, as, for instance, from the alisphenoids. The tympanic ring has been held to be the equivalent of the quadrate. It is more probably the quadrato-jugal.[16] [Illustration] FIG. 17.--A, First thoracic skeletal segment for comparison with B, fifth cervical vertebra (Man), _b.v._ Body of vertebra; _c_, first thoracic rib; _c_', cervical rib (which has become united with the transverse process, _tr_), the two enclosing the costo-transverse foramen (_f.c.t_); _st_, sternum; _zy_, articular process of the arch (zygapophysis). (From Wiedersheim's _Structure of Man_.) RIBS.--All mammals are furnished with ribs, of which the number of pairs differs considerably from group to group, or it may be even from species to species. The ribs are attached as a rule by two heads, of which one, the capitulum, arises as a rule between two centra of successive vertebrae. The other, the tuberculum, springs from the transverse process. Only in the Monotremes {30} are there ribs with but one, the capitular, head. In the posterior part of the series the two heads often gradually coalesce, so that there comes to be but one, the capitular, head. The Whales also, at least the Whalebone Whales, are exceptional in possessing but one head to the ribs, which is the capitular. The first rib joins the sternum below, and a variable number after this have the same attachment. There are always a number of ribs, sometimes called floating ribs, which have no sternal attachment. In the Whalebone Whales it is the first rib alone which is so attached. As a rule, to which the Whales mentioned are again an exception, the rib is divided into at least two regions--the vertebral portion which is always ossified, and the sternal moiety which is usually cartilaginous. This is, however, often very short in the first rib. They are, however, ossified in the Armadillos and in some other animals. Between the vertebral and sternal portions an intermediate tract is separated off and ossified in the Monotremata. The ribs of existing mammals belong only to the dorsal region of the vertebral column, but there are traces of lumbar ribs and also of cervical ribs. In the Monotremata, indeed, these latter {31} are persistently free for a very long period, and in some cases never become ankylosed with their vertebrae. But it should be noted that in this group there is no approximation to the state of affairs which exists in many