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 (Beztr. Pal. Oster-Ung., vol. xiii. p. 4, 
1901), to whom naturalists are indebted for these interesting 
researches. 
In their earliest stage of development the toothed whales 
