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392 : DENTAL ANATOMY. —

little importance can be attached to the dicondylian character of the
skull, from the fact that there is in certain members of this group a
double articular surface on the occipital bone for the atlas vertebra.
The only osteological character left in which the Batrachia resemble
the Mammalia most is that of the quadrate articulation ; which resem-
blance is somewhat counterbalanced by the approaches to the distinctive
peculiarities of the mammalian dentition found only in the Theromorpha.
The condition of the arterial system must remain inferential for this
group, since it became extinct, so far as we now know, at the close
of the Permian Period. Upon the wdiole, I am disposed to think that
there are quite as many reasons to regard the theromorph Reptilia
as the ancestors of the mammal as there are to regard in the same
light any of the Batrachia so far discovered.
Accepting the '* placoid scale " or the " dermal denticle " as the struc-
ture from which all teeth w-ere primarily derived, we have, as charac-
ters of a primitive dentition, the following : (1) the conical form ; (2)
increased number ; (3) frequent and almost endless succession. These
conditions we have fulfilled in many of the sharks. The next step in
specialization consists in the fusion of the basal osseous plates of the
'' dermal denticles " to form the maxillary and dentary bones, to which
the teeth become attached by ankylosis. This, we have already seen,
obtains in a majority of the fishes, and is associated largely with the
simple conical form. In the Batrachia the conical form, this mode of
attachment, as well as the succession, are closely adhered to, but the
individual teeth are reduced in number. In certain of the Reptilia e. g.
Theromorpha—another advance is made in the implantation of the teeth
in distinct sockets, wath a disposition to form more than one root or
fang. There are still, however, many successive sets of teeth developed.
Lastly, in the INIammalia the teeth are generally greatly reduced in
number ; they are always implanted by one or more roots in a distinct
socket, and there are never more than two sets developed, the second
of which is only partially complete ; they are also, as a general rule,
of a complex nature and show a wide departure from the primitive
cone.
In searching, therefore, for a primitive or generalized mammalian den-
tition, the most important point to be taken into consideration is the
following : numerous single-rooted teeth, confined to the maxillary and
mandibular bones, implanted in distinct sockets, with a complete devel-
opment of one or more successive sets. It is possible, even probable,
that this stage in tooth-development was reached in the ancestors of the
Mammalia before they assumed their distinctive characteristics as such
;
but the nearer any approach is made to this condition on the part of the
mammal, in that proportion it may be regarded as primitive in its den-
tal organization.
Having already spoken of the probable origin of the Mammalia, it
now remains to give a brief synopsis of their classification before pro-
ceeding to a detailed description of their teeth. The arrangement here
adopted is, with some modification, the one which has been proposed by
Prof. E. D. Cope, and is based upon a study of both fossil and recent
forms