532 BALENIDA. 
Fig. 223. 
Tympanic bone of Balena gibbosa, § nat. size. Felixstow. 
A third form of tympanic bone differs from the first in 
the shorter and more conyex form of the involuted part 
(a), the anterior end of which is divided from the anterior 
end of the cavity by a concave border one inch in extent ; 
the internal border of the involuted convexity is also better 
defined than in Bal. affinis; but the overarching wall (4) 
begins to rise close to it, divided from it only by a deep 
and narrow rugged fissure, instead of by a broad and gently 
concave tract, as in Bal. definita. 
Both the outer and under surfaces of these specimens 
are more rounded than in the two preceding species; but 
being more mutilated and water-worn, the differences 
of the external parts of the bone are of less value. The 
characters above specified, which are furnished by the 
involuted convexity, are decisive as to the specific distine- 
tion of the present fossil tympanic bone, and the third 
species of extinct whale, so indicated, I have proposed to 
call Balana gibbosa. 
