Existine Cetaceans are divided into two great primary groups, the
one comprising the true, or whalebone whales, in which the place of teeth is
taken by baleen or " whalebone," and the toothed whales, characterized by the
presence of functional teeth, at least in the lower jaw. These two groups differ
from one another in many important respects, and if they are derived from a
single stock, their common ancestor must have existed at a comparatively remote
epoch. Dr. Kukenthal is, however, of opinion, that the whalebone and the toothed
whales have originated independently of one another from totally distinct groups
of terrestrial mammals. If this view be ultimately maintained, it will be
evident that the Cetacean order, as at present constituted, is a heterogeneous
group; while we should have a most remarkable instance of the power of
adaptation to a particular mode of life of producing similarity in form.
The whole of the remainder of the existing Cetaceans form a group distinguished
from the preceding one by the absence of whalebone, and the presence of
permanent teeth in at least the lower jaw. This group, comprising existing
families, is known as the Odontoceti, or Toothed Cetaceans, in contradistinction
to the Mystacoceti, or Whalebone Whales.
In addition to the presence of teeth, the group is also characterized by the
following distinctive features. The two nostrils unite before they reach the
surface, and thus open by a single aperture, which usually takes the form of a
crescentic valvular slit placed transversely to the long axis of the head. In
the skull the bones of the upper surface are arranged unsymmetrically on the
sides; the nasal bones in existing forms being reduced to nodules, taking no
share in roofing over the cavity of the nostrils. The two branches of the lower
jaw are nearly straight, and of great vertical extent behind, while in front
they come in contact with one another by flattened surfaces of larger or smaller
extent, as is ordinarily the case among mammals. The ribs are more firmly joined
to the rest of the skeleton than in the whalebone whales, several of the
anterior pairs articulating with the bodies as well as with the horizontal
transverse processes of the vertebrae, while below they are joined to the
breast-bone by the intervention of so-called chest-ribs; the breast-bone itself
usually consisting of several distinct portions, placed one behind the other. In
all cases the flippers have five digits. --
The Royal Natural History: Mammals, birds By Richard Lydekker - 1895 Edited by
Stanley L. Klos 1999
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