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 
(Neophocaena). 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 Sgualodon the molars 
alone retain the double-rooted character of Zeuglodon, and 
