CETOTOLITES. 529 
As none of the fossils in question have been found in 
situ, with any part of the cranium, their size in proportion 
to that of the animal cannot be judged of; but in the 
specimens that have been least injured and water-worn, 
the inferior surface shows the flattened or gently concavo- 
convex undulation which characterises the tympanic bone 
in true Baleenze. 
In regard to the differences which are observable in the 
tympanic bones of the two known species of Balena (Bal. 
mysticetus, and Bal. australis, capensis, or antarctica,) 
Cuvier merely observes that “though slight, they add to 
the motives which led him to believe the Arctic whale and 
that of the Cape to be specifically distinct.” This remark 
at least encourages us to regard the characters derivable 
from the tympanic bone as sufficiently determinate to be ¢ 
guide in the discrimination of species; and with this con- 
viction I have proceeded to compare the fossils in question 
with the recent tympanic bones of the two above-cited 
existing species of Baleena. 
In these the thick convex involuted portion of the tym- 
panic bone is slightly and unequally raised above the level 
of the cavity formed by the over-arching wall, but in the 
Bal. antarctica it gradually decreases in thickness to the 
anterior or Eustachian angle; while in the Bal. mysticetus 
the thicker posterior part is defined by an indentation from 
the thinner anterior part. In both species the thinner part 
of the convex border is distinctly continued to the anterior 
limit of the cavity; in both the extent of the involuted 
convexity, inwards, is not well defined, but it gradually 
subsides, and the convexity is exchanged for the concave 
curve of the overarching wall. The inner surface of this 
wall is very rugged near the involuted part. I purposely 
omit the mention of the slight differences in other parts of 
M M 
