Nearly allied to the preceding is the likewise aquatic
order of the Whales and Dolphins (_Cetaceans_), in which the body
is also fish-like, the hind-limbs are wanting, the fore-limbs are
converted into powerful "flippers" or swimming-paddles, and the
terminal extremity of the body is furnished with a horizontal,
tail-fin. Many existing Cetaceans (such as the Whalebone Whales)
have no true teeth; but others (Dolphins, Porpoises, Sperm Whales)
possess simple conical teeth. In strata of Eocene age, however, we
find a singular group of Whales, constituting the genus _Zeuglodon
(fig. 228), in which the teeth differed from those of all existing
forms in being of two kinds,--the front ones being conical incisors,
whilst the back teeth or molars have serrated triangular crowns,
and are inserted in the jaw by two roots. Each molar (fig. 228,
A) looks as if it were composed of two separate teeth united on
one side by their crowns; and it is this peculiarity which is
expressed by the generic name (Gr. _zeugle_, a yoke; _odous_,
tooth). The best-known species of the genus is the _Zeuglodon
cetoides_ of Owen, which attained a length of seventy feet. Remains
of these gigantic Whales are very common in the "Jackson Beds" of
the Southern United States. So common are they that, according
to Dana, "the large vertebrae, some of them a foot and a half
long and a foot in diameter, were formerly so abundant over the
country, in Alabama, that they were used for making walls, or
were burned to rid the fields of them.
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