312 MOSTLY MAMMALS 



were full armoured. The object of the armour was as a 

 defence against enemies, such as sharks, such an armour 

 being also very valuable to animals exposed to the force 

 of a strong surf on rocky shores. As the creatures took 

 more and more to an aquatic life, the acquisition of greater 

 speed would be of greater value to them, and this would 

 be accomplished by diminishing the specific gravity and 

 friction of the body, the shortening of the extremities 

 and the development of a caudal fin to serve as the sole 

 instrument of locomotion. 



Accordingly the armour would very soon be lost by the 

 pelagic cetaceans in order to diminish friction and lighten 

 the specific gravity. Only among certain types, which 

 diverged at an early epoch from the ancestral stock and 

 took to a fluviatile or estuarine life, did vestiges of the 

 armour remain, while the dorsal fin remained undeveloped 

 (Neophociiena). That in this form, as well as in the closely 

 allied true porpoises {Phocaena), we have the most primitive 

 type of living toothed whales, is confirmed by the nature 

 of the dentition as well as by the circumstance that in this 

 group alone the premaxilla is toothed. The relation of the 

 interparietal to the parietal bones of the skull is likewise 

 confirmatory of the antiquity of the porpoises. 



It may be added that Zeuglodon differs from modern 

 cetaceans by the characters of its teeth, those of the 

 lateral series being double-rooted and having compressed 

 and serrated crowns, distantly recalling those of the leopard- 

 seal. Between Zeuglodon and the shark-toothed dolphins 

 {Squalodon) the gap is very great, but still one which 

 might readily be bridged were the missing links forth- 

 coming ; and as it is, the molars of the one type seem 

 derivable from those of the other. In Squalodon the molars 

 alone retain the double-rooted character of Zeuglodon, and 



