ORGAN OF HEARING IN MAMMALIA. 



231 



whole length of the handle, affect, for the most part, a trans- 

 verse course. The stapes has a wide vacuity and slender crura, 

 and in many Rodents (Squirrels, Cavies, Marmots, fig. 173, E) it 

 is traversed, as in Moles, by the bony canal of a vessel and nerve. 

 The ear-conch shows a wide range of variety, from some swim- 

 mers ( Castor) and burrowers (Mole-rat, fig. 174) where it is hardly 

 visible, to the Flying Squirrels (vol. ii. fig. 154), Jerboas and 

 Hares, where the ears are conspicuous appendages to the head. 



The Sloths contrast with the Rodents in the degree in which 

 they enjoy the sense of hearing : the conch is rudimental ; there 



174 



Uiitliyergus maritimus. 



is no bony meatus ; the tympanic is reduced to its function as 

 the frame of the drum-head, and lon^ retains its individuality. 



t_j v 



The stapes is small and imperforate, in the two-toed species, 

 fig. 173, F. The crura of the incus are thick, of equal length, 

 very divergent. The handle of the malleus is bent at the middle; 

 it is short, as is also its process. The tympanic cavity extends 

 into the squamosal and pterygoid. 



In the Armadillos (Dasypus) the meatus interims is subspiral ; 

 the cochlea projects into the tympanic cavity : this is large, but 

 owes little to the tympanic bone. The malleus is bent, almost as 

 in Monotremes, completing the circle for the drum-membrane, 

 and expanding for its attachment thereto : the part articulated 

 with the incus is very broad and flat. The ear-conch is con- 

 tracted and tubular at its base ; but expands to a length of nearly 

 two inches and a breadth of one inch in Dasypus Peba, in which 

 the apex is rounded off. In Orycteropus, also, the external ears 

 are very large for a burrower. In the true anteaters (Myrme- 

 cophagci) they are much smaller : the tympanic bone retains its 

 freedom and is chiefly subservient to the support of the drum- 



