ARMOUR-CLAD WHALES 311 



as well as some portion of the back, was covered with a 

 complete tesselated armour of bony plates. 



The majority of the living toothed whales (inclusive of 

 porpoises and dolphins) are furnished with a dorsal fin, 

 and it is therefore reasonable to suppose (apart from the 

 evidence of the specimen just referred to) that Zeuglodon 

 was similarly provided ; and if this be so, that cetacean 

 was evidently a pelagic creature. For the function of a 

 dorsal fin is to act as a kind of keel in maintaining the 

 balance of the body, this appendage being most developed 

 in purely pelagic cetaceans like the killer, while in littoral 

 or fluviatile forms such as the narwhal, the white whale, 

 and the Japanese porpoise, it is either small or wanting. 

 It is, further, noticeable that cetaceans with pointed muzzles 

 (of which Zeuglodon is one) nearly always have a larger 

 back-fin than those in which the muzzle is short and 

 rounded. In the whalebone bones, among which the 

 dorsal fin is either small or wanting, its function may be 

 discharged by the keel on the middle of the upper jaw, 

 or, owing to corporeal bulk, no such function is required 

 at all. 



If, then, we are right in regarding Zeuglodon as a pelagic 

 cetacean, it is evident that it could not have been completely 

 armoured, but that such armour as it retained was merely 

 a survival from a fully armoured non-pelagic ancestor. For 

 it is almost impossible to believe, if they were armoured at 

 all, that the ancestral form was not invested in a complete 

 panoply, at least on the dorsal region. 



The whole argument is tersely summed up as follows 

 by Dr. O. Abel {Beitr. Pal. Osier.- Ung., vol. xiii. p. 4, 

 1901), to whom naturalists are indebted for these interesting 

 researches. 



In their earliest stage of development the toothed whales 



