228 ANATOMY OF VERTEBRATES. 



afforded by the successive steps by which the ordinary mamma- 

 lian condition of the otosteals is attained. In most, as in Perar 

 171 meles, the stapes is still columelliform, fig. 171, 



/l^ggx e : its base oval, supported on an imperforate 



a v ^T^& stem » * ts a P ex more expanded than in Mono- 

 fp0 tremes, sending off the process, d, and develop- 

 d^f/ ing the articular cup for the malleus, a-c. This 

 \ representation of ' incus ' begins, as in Echidna, 

 •~«5lP ky a separate ossification. In Macropus, it 

 more commonly retains its individuality, and 



Otosteals, Pcramcles. , n \ *,c> • , i (* . ^ 



the stapes, tig. 172, is minutely perforate above 

 the disc : however, in some instances, it also shows the process, d. 

 The resemblance of the malleus, fig. 171, to the ' cartilago 

 columella)' of birds is instructively close in most 

 marsupials : but the parts called the f head,' c, 

 b ( body,' b, and ( handle,' «, are definable. The 

 ^^ largest proportional external ears are those of the 

 Pcrameles lagotis, the shortest those of the Wom- 

 bat. The tympanic cavity in Perameles is very extensive, but is 

 formed by the sphenoid and petrosal bones ; the tympanic bone 

 is limited to the function of supporting the ear-drum, and forming 

 the internal commencement of the meatus auditorius externus. 

 The internal extremity of the tympanic cylinder projects ob- 

 liquely into the posterior and outer part of the sphenoidal bulla. 

 In many other marsupials the tympanic is prolonged into a 

 bony support of more or less of the external ear-passage, the 

 extent and direction of which are noted in vol. ii. p. 340 : the 

 species in which the tympanic cavity is supplemented by excava- 

 tions in the squamosal are also there mentioned. 



It is interesting to find, in some Bats, a retention of the colu- 

 melliform confluence of stapes and incus, as in fig. 173, a ( Ves- 

 pertilio noctula). All insectivorous Cheiroptera likewise show the 

 semicircular canals projecting from the rest of the acoustic bony 

 capsule, which is relatively large and free. The cochlea, however, 

 departs far from the Bird-type, being of unusual relative size, and 

 in some species describing more than three turns: divided as 

 usual into the two scalar, of which the tympanic one, as in Whales, 

 is much the largest. The divisions of the meatus internus for the 

 cochlear and vestibular branches of the nerve are unusually deep 

 and distinct. The tympanic is here mainly subservient to the 

 support of the drum-membrane : it is deeply sunk into the tym- 

 panic cavity, and Aery concave outwardly. One branch of the 

 Btapes is thicker than the other; the two crura of the incus are 



