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 Balsenae. 



In regard to the differences which are observable in the 

 tympanic bones of the two known species of Balsena (Bal. 

 mysticetus, and Bal. austratis, 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 a 

 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 Balsena. 



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 



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