223 ANATOMY OF VERTEBRATES. 



afforded by the successive steps by which 1he ordinary mamma- 

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

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



e : its base oval, supported on an imperforate 

 stem ; its apex more expanded than in Mono- 

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

 ing the articular cup for the malleus, a-c. This 

 representation of ' incus ' begins, as in Echidna 9 

 by a separate ossification. In Macropus, it 

 more commonly retains its individuality, and 



Otosteals, Peranaeles. , / *-i*n - i f 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 



columellse ' of birds is instructively close in most 



1 7^ 



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

 6 body,' b, and ' handle,' , are definable. The 

 largest proportional external ears are those of the 

 Perumeles 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 fio*. 173, A ( Ves- 



i ' O \ 



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 scala3, of which the tympanic one, as in Whales, 

 is much the largest. The divisions of the meatus interims 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 very concave outwardly. One branch of the 

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



