28 MAMMALIA. 



Bears, Cats, Dogs, and probably the Carnivora in general, have three 

 complete coils ; the Pig, the Squirrel, and other Rodentia, have 

 nearly 4 ; in Cselogenys Paca nearly 5 turns are met with ; in the 

 Ornithorynchus and Echidna, on the contrary, the cochlea has only a 

 half coil, and rather represents a semilunar cone, comparable to 

 the cochlea of a Bird ; it has, however, a modiolus and two scalse. 

 The size and form of the two fenestrac vary remarkably ; in the 

 Seal, for example, the foramen rotundum is three times larger than 

 the f. ovale. The tympanic cavity offers in the class Mammalia the 

 greatest differences of all. In Man, and in the Apes, it is completely 

 concealed in the petrous bone ; in the remaining orders, on the con- 

 trary, we find a peculiar tympanic bone which exhibits great diver- 

 sities in the several orders. In the Cetacea it is large, and hard as 

 ivory, remains completely separated from the much smaller petrous 

 bone, and like it, is only united by ligament to the skull. In the 

 Ruminantia, the tympanum is angular and irregular, in the Ox very 

 cellular, and the Sheep and Goat spacious and devoid of cells. The 

 Horse and Pig have a cellular tympanum. In many Rodentia 

 and Carnivora, where it usually swells out into a bony bulla (very 

 large in Dipus), the tympanum remains at least a very long time 

 separated, or is united by means of a suture, which sometimes 

 disappears at a later period ; externally there is often appended or 

 united to the tympanum a bony tympanic ring, which is not always 

 complete above. The tympanic cavity extends occasionally into the 

 other cavities of adjacent bones, e. g. in the Sloth, into the zygo- 

 matic arch. The membrana tympani is (except in the Cetacea) 

 drawn in a somewhat funnel-shape inward, lies sometimes, as in 

 the Mole, nearly horizontal, or approximates that position, as in 

 many Carnivora and Edentata, while elsewhere it stands more 

 perpendicularly, as in Man. The Eustachian tube is partly bony, 

 partly cartilaginous, and always opens by a peculiar orifice behind 

 the nasal passages within the fauces. In the Horse and Ass the 

 Eustachian tube, upon either side, is always united with a mem- 

 branous oviform purse or air-sac, which lies within the fauces, 

 beneath the occipital bone, and is formed of mucous membrane, the 

 sacs of either side being contiguous. The three auditory ossicles 

 can in general be distinguished, namely, the malleus, the incus, 

 with the os orbiculare and the stapes, and although their forms un- 

 dergo considerable changes, we frequently recognise in them the 

 human type, as in the Apes of the Old World. In those of the 

 New World their form varies more ; the opening into the stapes for 



