230 ANATOMY OF VERTEBRATES. 



a wide aperture ; it is as large as the incus, the attached crus of 

 which is short, its free one long : the malleus equals both bones, 

 and its head and body, as in Shrews, are unusually expanded. In 

 Centetes it is less broad, with a longer process and shorter and 

 thicker handle. The membrana tympani is almost horizontal ; 

 there is no bony meatus externus. The ear-conch is short, broad, 

 and rounded : two of its muscles are derived from the strongly 

 developed dorsal panniculus carnosus, fig. 7. In the Tenrec the 

 tympanic extends into a short ( meatus externus.' 



In the Rat (Mus decumanus) the orifices for the cochlear and 

 vestibular divisions of the acoustic nerve open separately on the 

 petrosal surface, not into a common ' meatus internus.' In sec- 

 tions of the cranium of some Rodents I observed that the tym- 

 panic cavity was divided by a horizontal partition into an upper 

 and lower compartment, intercommunicating, in the Porcupine, 

 posteriorly above the membrana tympani ; this is situated in the 

 lower compartment, the external meatus terminating in a narrow 

 oblique slit at its upper part. In the beaver the upper com- 

 partment of the tympanum is much smaller; the bony meatus 

 contracts to a transverse slit as it approaches the membrana tym- 

 pani, the plane of which is almost parallel with that of the meatus 

 itself : from the membrane the bony meatus extends outward and 

 curves forward and a little upward. 1 In the Paca ( Ccelogenys) the 

 horizontal septum divides only the anterior half of the tympanic 

 bulla into an upper and lower compartment, the meatus termi- 

 nating, as usual, in the latter. The tympanic cavity is remarkably 

 developed in most members of the present active timid order : it 

 is enormous in Ctenomys. 2 In the Chinchilla (Lagotis) the mastoid 

 portion rises to the upper surface of the cranium, where it is 

 girt by a slender band of the combined superoccipital and squa- 

 mosal : the petrosal part of the tympanic bulla describes a curve 

 downward and backward circumscribing; a large foramen which 

 opens into the bulla beneath the meatus auditorius externus. 

 This is long, wide, funnel-shaped, with the outlet obliquely trun- 

 cate and directed upward and a little backward. In the Capy- 

 bara the bony meatus externus is unusually contracted, is cleft 

 below, and bounded there by two small tuberosities. In the 

 Hare the meatal part of the tympanic is long and ascends ob- 

 liquely backward from the frame of the drum-membrane. This 

 is a long ellipse ; the handle of the malleus extends from above 

 down its long axis to about one-fourth from the lower border ; 

 the fibres of the ' membrana propria,' diverging from nearly the 



1 xliv. Nos. 2044, 2093, 2166. 2 xliv. p. 365, no. 2012. 



