AQUATIC MAMMALS 



section of the peduncle varies greatly according to the habits to a large 

 extent the speed of the animal. In some of the slower sorts of whales 

 (balaenids, gray whale, etc.) this may be almost circular and fairly el- 

 liptic. I believe that the height is invariably greater than the width, 

 however. At the other extreme in peduncular shape are the rorquals. 

 In a finback (Balaenoptera physalus) of 65 feet the peduncle directly 

 anterior to the flukes measured approximately one foot in width and four 

 feet in height. Instead of being purely elliptic the peduncle was quite 

 sharply keeled above and below. It is obvious that such a shape would 

 prove of very great economic benefit in reducing water resistance to the 

 minimum as the animal elevates and depresses the tail while swimming. 

 Sections cut from the peduncle show that in a specimen of this size the 

 dorsal and ventral 8 inches of the respective keels are composed of the 

 same sort of fibrous tissue as the flukes. So it is obvious, I think, that 

 these narrow, angular, peduncular keels have been built up above and 

 below the great spinal tendons for the sole function of reducing water 

 resistance during swimming, which is just what the flukes have done to 

 increase resistance. But such contradictory situations are frequently en- 

 countered in any study of specialized organisms. 



Save for the fibrous keels as described above, the shape of the pe- 

 duncle of the Cetacea is dependent upon details of the vertebrae of this 

 region, and therefore upon the character of the musculature concerned. 

 It is clear that in those sorts of whales with relatively broad peduncles lo- 

 comotion is retarded by just so much, and a broad peduncle can be of 

 use only to give the caudal muscles greater leverage in lashing the tail 

 laterally a movement that cannot be of very great importance to the 

 animal. With a peduncle that is relatively very high or deep a whale is 

 equipped not only to elevate and depress the peduncle with the minimum 

 of water resistance, but also to secure, by means of long spinous proces- 

 ses and chevron bones, increased leverage by which the tail may be 

 raised and lowered with greater power or greater ease. 



The caudal vertebrae of living Cetacea vary in number from 16 in 

 Neobalaena to 32 (or possibly more) in Steno and Lagenorhynchus, and 

 the variation in height of spines and width of transverse processes is 

 great. Prezygapophyses are present in mysticetes, and in most odon- 

 tocetes, but in some forms (as Grampus) of the latter these are sup- 

 pressed. Chevron bones are present, taking the place below that is 

 filled above the column by the spinous processes. Their development 

 corresponds to that of the spines, the latter invariably being slightly 

 longer. As Flower has said regarding the caudal vertebrae, in passing 



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