Page 484 - My FlipBook

494 DENTAL ANAT03IY.

the crowns of these teeth, and never having seen a specimen myself, I
am at present unable to say just M'liat the pattern of the crown is. From
the best information at my command I suppose it to be somewhat after
the style of a modified tuberculo-sectorial. I further do not know
\vhether the succession has been observed, and whether a proper distri-
bution of the molars and premolars expressed in the above formula has
been made; but, judging from the condition in marsupials generally, I
am induced to believe it to be correct. It is so given by Owen and
Waterhouse.
Some fragmentary remains, consisting principally of jaws and isolated
teeth, of a number of small mammals have been discovered from time
to time in the Jurassic and Triassic deposits of this country, Europe,
and South Africa, in which the teeth behind the canines reach as high
a number as twelve in each lower jaw in some species. These are some-
what arbitrarily divided into an equal number of molars and premolars,
but whether any or all of them had deciduous predecessors is not
known. The reason for this division is that the first six behind the
canine are premolariform in shape, while the others possess a number of
sharp cusps. They have been referred to the marsupials and assigned a
position near to Jli/rmecobins, but until their osteology is better known
this is doubtful. Inasmuch as they are the oldest known mammals, we
should anticipate on a priori grounds that they really belong to the
monotremes instead of the marsupials. The great number of teeth cer-
tainly constitutes an approach to the Reptilia, and if they possessed a
complete development of a second set, which is not at all improbable,
the transition between reptile and mammal would be in a measure com-
plete as regards the teeth.
Another strange and remarkal)le genus, P/ar/inulax, together with a
number of allies, comes from these ancient horizons. In this animal
the molar pattern is complex for so early a representative of the Mam-
malia, and is difficult to understand. In the loAver jaw of Plagiaulax
there are seven teeth, of which the first is large, curved, and pointed,
and is probably an incisor. This is followed after a considerable space
by four teeth, all of which, except the first, are implanted by two roots
and increase gradually in size. Their crowns are terminated superiorlv
by a wedge-shaped crest directed antero-posteriorly, which is rendered
subserrate by the presence of a number of oblique vertical grooves.
Behind tliese are two smaller teeth with tubercular crowns, which
have been supposed to represent true molars.
The remaining marsupials whicli are really known to be such are
divisible into the Polj/protodontia, or those of predaceous habits, having
many incisors, and the Diprofodontia, vegetable feeders, having onlv
two incisors, in the lower jaw. As far as dental characters go, thev all
agree in the possession of fi)ur true molars; there are never more than
three premolars, and the deciduous molars, ^vhich are succeeded at a
comparatively late jx'riod by tlie last premolars, are reduced to one in
eacli jaw. This, therefore, furnishes another example wherein the defi-
nition of a premolar is violated.
Three families are included in the polyprotodont division, one of
which, the opossums, is confined to North and South America, and the