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THE AMERICAN NA TURALIST. [Vol. XXXVII. 



istic cylindrical form, there being little or none of the lateral com- 

 pression such as is common among land forms, and this seems 

 to be the first step in the enlargement of the chest capacity, as 

 it is found in Pinnipedia as well as in the Sirenia and Cetacea. 

 The ribs at first tend to become highly arched dorsally and then 

 to move upward in their point of attachment from the centra to 



found in the Pinnipedia and its culmination is seen in the whale- 

 bone whales, Balaenoidea, where all the ribs are attached only 

 to the transverse processes of the vertebrae. Possibly this is of 

 service in equilibration as the lungs can take a more dorsal posi- 

 tion. Accompanying these changes the diaphragm becomes 

 much more oblique and much more strongly muscular, undoubt- 

 edly giving greater control over the chest capacity in the peculiar 

 conditions of respiration necessarily accompanying aquatic life. 



Perhaps the most striking externa! adaptation to aquatic life 

 is the assumption of "fins " for use in swimming. Many of the 

 Cetacea have developed a fleshy dorsal fin which undoubtedly 

 serves the same purpose as the similar organ among the fishes 



expanded caudal fin supported by a dense framework of connec- 

 tive tissue and used as a propeller in swimming. This organ 

 differs from that of the fishes in being expanded laterally instead 

 of vertically, and this arrangement of the fin permits the animal 



noticeable in these animals when they are swimming at the sur- 

 face. The flukes of the tail are said to be capable of a some- 

 what rotary sweep like the blades of a screw propeller at each 

 stroke of the tail. It is a noteworthy fact that nearly all 

 aquatic mammals have this dorso-ventral flattening of the tail, 

 the only exceptions being Potamogale, Myogale and the musk- 



