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Ch. IX.] 



AT SUCCESSIVE GEOLOGICAL PERIODS. 



103 



Absence of cetacea in secondary rocks. — But there is a 

 negative fact of great significance which seems more than 

 any other to render it highly improbable that we shall 

 ever find air-breathers of the highest class in any of the 

 primary strata, or in any of the older members of the secon- 

 dary series. This fact is the absence hitherto of all bones 

 of cetacea among the numerous remains of fossil vertebrata 

 entombed in rocks older than the Eocene- Cetacean bones 

 are of rare occurrence in the Lower Tertiary formations of 

 Europe, the only instance in Great Britain being a species 



from the London clay, and the position even 



Monodon 



specimen 



middle 



Alabama 



s x s 



mammal 



means 



of uncommon occurrence.* A series of anchylosed 

 cervical vertebrae of a whale found near Ely, in Cambridge- 

 shire, is supposed by Professor Sedgwick to have been derived 



some member 



it was not obtained 



from a rock in situ, but from argillaceous drift into which 



must 



tion still unsettled. The dimensions of the cetacea in general 



them 



selves on the notice of collectors had they been entombed 



mud 



formations where the skeletons of huge reptiles are so con- 

 spicuous. The ichthyosaurs and other carnivorous saurians 

 seem formerly to have played the part now assigned to the 

 cetacea in the economy of nature ; and if we assume this to 

 have been the case, it seems probable that the placental 

 mammalia, if they existed at all before the Tertiary period, 



were at least extremely scarce. 



Successive appearance in chronological order of 

 classes of mammalia of higher and higher grade.— 

 cation of mammalia, founded on the modification of their cere- 

 bral structure, Professor Owen has assigned the lowest place 



In a classifi- 



* The supposed cetaceans of the lately been ascertained by him to be of 

 cretaceous rocks, which I formerly cited Miocene date.— Leidy, Keptiles of the 

 on the authority of Dr. Liedy, have Chalk. 



M 2 



