The ear always contains a cavity called the tympanum, or drum, which communicates with the mouth by the Eustachian tube; the cavity itself is closed externally by a membrane called the membrana tympani, and contains a chain of four little bones, named the incus or anvil, malleus or hammer, the os orbiculare or circular bone, and the stapes or stirrup; a vestibule, on the entrance of which rests the stapes, and which communicates with three semicircular canals; and finally a cochlea, which terminates by one canal in the vestibule and by the other in the tympanum. Their cranium is subdivided into three portions; the anterior is formed by the two frontal and ethmoidal bones, the middle by the two ossa parietalia and the os ethmoides, and the posterior by the os occipitis. Between the ossa parietalia, the sphenoidalis, and the os occipitis are interposed the two temporal bones, part of which belong properly to the face. Their face consists of two maxillary bones, between which pass the nostrils; the two intermaxillaries are situated before and the two ossa palati behind them; between these descends the vomer, a bony process of the os ethmoides; at the entrance of the nasal canal are placed the ossa nasi; to its external parietes adhere the inferior turbinated bones, the superior ones which occupy its upper and posterior portion belonging to the os ethmoides. The jugal or cheek bone unites the maxillary to the temporal bone on each side, and frequently to the os frontis; finally the os unguis and pars plana of the ethmoid bone occupy the internal angle of the orbit, and sometimes a part of the cheek. In the embryo state these bones also are much more subdivided. Their tongue is always fleshy, connected with a bone called the hyoides, which is composed of several pieces, and suspended from the cranium by ligaments. Their lungs, two in number, divided into lobes and composed of an infinite number of cells, are always inclosed, without any adhesion, in a cavity formed by the ribs and diaphragm and lined by the pleura; the organ of voice is always at the upper extremity of the trachea; a fleshy curtain, called the velum palati, establishes a direct communication between their larynx and nasal canal. Their residence on the surface of the earth rendering them less exposed to the alternations of cold and heat, their tegument, the hair, is but moderately thick, and in such as inhabit warm climates even that is rare. The Cetacea, which live exclusively in water, are the only ones that are altogether deprived of it. The young are nourished for some time after birth by a fluid (milk) peculiar to animals of this class, which is produced by the mammæ at the time of parturition, and continues to be so long as is necessary. It is from the mammæ that this class derives its name, and being a character peculiar to it, they distinguish it better than any other that is external.