Page:Mammalia (Beddard).djvu/49



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.

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