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414 DENTAL ANATOMY.

In tlie second sub-order (Denticetc) no baleen plates are developed ;
teeth are always present, and are more or less persistent. They are
implanted by single roots, and are in some instances very numerous.
No second dentition has ever been observed in any member of this
group, and they are, so far as known, truly monophyodont. In the
common porpoise of our coast (Delphinus chymene), which is an average
example of this sub-order, the teeth are about ninety-four in number,
and are lodged in the premaxillary, maxillary, and mandibular bones.
They are implanted by single sligh'tly enlarged fangs in ill-defined
sockets incompletely partitioned otf from each other, and in what at
first sight seems to be a wide-open groove. Their crowns taper grad-
ually to a sharp point, which is strongly incurved. The first two teeth
in the upper ja\v are small and implanted in the premaxillary bone,
Avliich furnishes a very small part of the dentigerous border of the upper
jaw. Behind these the maxillary teeth rapidly increase in size up to the
seventh or eighth tooth, after which they continue to the fifteenth or six-
teenth almost equal in size, and then gradually diminish in size toward
the posterior j^art of the jaw. The teeth of the inferior series are like
those of the upper, except that the posterior ones are more robust.
The jaws are remarkable for their great length and narrowness, and the
arrangement of the bones of the face when compared with other mam-
mals is also peculiar. The coronoid process of the lower jaw is obsolete.
In other members of the DelpJiinidcv—the dolphin, for exam])le—the
teeth are often as many two hundred, the greatest number exhibited by
any mammal, or they may be reduced to a single functional tooth, as in
the narwhal [3Ionodon). In this latter species four teeth are found in a
foetal state, but the two lateral ones are lost or absorbed before birth.
In the male narwhal the left of the two anterior ones, which is placed
in the premaxillary bone^ grows from a persistent pulp and attains a
length of ten or twelve feet. This formidable tusk is almost straight,
and is marked by spiral ridges which wind forward from left to right.
The corresponding tooth of the opposite side sometimes reaches a devel-
opment equal to that of the left, but more frequently its growth is
arrested and it remains buried beneath the gum, as do both in the
female. Owing to this circumstance, it has been thought that it
serves as a sexual weapon similar to the antlers of the deer, but until
the habits of the animal are better known this explanation of its use
must remain conjectural.
The great bottle-nosed whale {Hyperoddon bidens) is, to all outward
appearances, edentulous, but careful examination reveals the presence of
two, sometimes four, well-calcified conical teeth in the front jiart of the
jaw, which remain more or less completely hidden by the gum. In
addition to these, there are usually twelve or thirteen small rudimentary
teeth imbedded in the gums of both jaws, which soon disappear.
In the sperm whale {Physdcr macrocephcdufi) the exposed and func-
tional teeth are confined to the lower jaw. These are about twenty-
seven in number in each ramus, loosely implanted in a wide-open gutter,
with the alveoli or sockets scarcely pci'ceptible. They are at first
sharply conical, but by attrition wear down into obtuse cones, biting
into pits or cavities in the gums of the upper jaw. In this jaw there