15 
1913-14.] The Auditory Organ in the Cetacea. 
and had evidently been invested by the tympanic membrane (fig. 3, Au), 
where it formed the cul-de-sac which projected into the auditory meatus ; 
Fig. 3. — Outer aspect of left tympano-petrous bones of Balcenoptera sibbaldi , natural size. A, 
anterior end, P, posterior end of tympanic ; Au, indicates the region adjoining the external 
auditory meatus covered by the tympanic membrane and the place of projection into meatus of 
tympanic membrane ; l , lip-like process of sinuous border with which malleus is fused ; M, 
head of malleus ; S, head of stapes in funnel-like depression on inner wall of tympanum ; L, 
labyrinthine part of petrous ; Pr, pre-otic, and Op, opisthotic parts of petrous cut across ; Ap, 
anterior, and Pp, posterior tympano-petrous peduncles. The incus is not figured, as it would 
have obscured the stapes. 
the smooth surface was bounded by a rough area, to which had doubtless 
been attached the deep end of the wall of the meatus. The sinuous border 
