284 • [Dec. 13, 



by Camper* in his usual sketchy style. Cuvier, who founds his 

 notice of the tympanic bones of the Cachalot on the same figures, 

 states that they most resemble those of the DelphiriidcE ; but are 

 less elongated and less bilobed posteriorly. The figures show still 

 more clearly that the tympanic cavity is continued freely forward 

 out of the anterior end of the bone, and terminates by a rela- 

 tively wider outlet than in the Delphinidce. 



K the idea thus given of the form of the tympanic bone of the 

 Cachalot be correct and conformable to nature, the comparison of 

 the Cetacean fossils becomes limited to the true whales {BalcenidcB), 

 in the few known species of wliich the distinctive characters of 

 the tympanic bones are aiForded by their relative size and the shape 

 of their inferior surface. 



In Balcenoptera the tympanic bones, according to Cuvier, are 

 very small in proportion to the head, and are equally convex at 

 their inferior surface. 



As none of the fossils in question have been found in situ, with 

 any part of the cranium, their size iia 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 flat- 

 tened or gently concavo-convex undulation which characterises the 

 tympanic bone in true BalcEnce. 



In regard to the differences which are observable in the tym- 

 panic bones of the two known species of Baltena {Bal. 7nysticetus, 

 and Bal. australis, capensis, or antarctico) Cuvier j" merely ob- 

 serves 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 cha- 

 racters derivable from the tympanic bone as sufficiently deter- 

 minate to be a guide in the distinguishing of species ; and with this 

 conviction I have proceeded to compare the fossils in question 

 with the recent tympanic bones of the two existing species of 

 BaliEna. 



In them the thick convex involuted portion of the tympanic 

 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 invo- 

 luted convexity, inwards, is not well defined, but it gradually sub- 

 sides, and the convexity is exchanged for the concave cui've of the 

 overarching wall. I purposely omit the mention of the slight 

 difference in other parts of the tympanic bone of the Balcena 

 mysticetus and antarctica, since the condition of the fossils would 

 not admit of the application of those differences in the determina- 

 tion of their affinities. 



* Anatomie des Cetaces, Pis. xxiii. xxv. 

 t Ossemens Fossiles, 4to., v. pt. i.' p. 376. 



