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OF THE GRINDERS. 87
the sides of the fang in the middle, thereby dividing the broad
and flat cavity into two ; and all along the outside of these
(and all the other flat fangs) there is a corresponding longitu-

two three-sided lobes, by an external groove. All the molars are curved
—in the upper jaw, the concavity of the curve is directed outwards ; in
the lower, inwards. The premolar is about half the size of a true molar.
Like the incisors, the premolars and molars of the Wombat have per-
sistent pulps ; they are therefore implanted by an undivided base. In
this respect, these teeth differ from the molars of all the other Marsupials,
whilst they resemble those of the dentigerous Bruta and herbivorous
Rodents.
The crowns of the molars in the order Insectivora are always
broader in the upper jaw than in the lower ; they also always present
several sharp points or cusps upon the working surface. y The dentition
of the Chrysochlore, or iridescent Mole of the Cape, is remarkable for
the number of the molar teeth, for their shape, and for their separation
by vacant intervals, as in many Reptiles. The number of the molar
series is p. \ ; \ m. -| ; £. This formula is based on the shape of the
teeth, as the facts relating to their vertical displacement and succession
are not known. The premolar in the upper jaw has a simple, com-
pressed, tricuspid crown. The crowns of the true molars are thin
plates, compressed antero-posteriorly, with two notches on the working
edge, and a longitudinal groove along the outer and thicker margin.
They are separated by intervals. In the lower jaw, the molars attain a
much greater height above the sockets ; they are separated by wider
intervals than those above, and the two series interlock when the mouth
is closed. In mastication, the anterior margin of the lower tooth works
against the posterior margin of the opposed upper tooth. The crowns
of the lower molars are long plates, compressed antero-posteriorly ; the
inner margin is the thickest, and is surmounted by two points. The
outer margin terminates in a single point, which is most prominent.
In an extinct small insectivore, the Spalacotherium, there were ten teeth
in the molar series on each side of the lower jaw ; of these, at least six
had tricuspid crowns, and bore a marked resemblance to the same teeth in
the Chrysochlore. In the Shrew Moles of America (Scalops) the number of
the molar series is reduced to p %\ l\ m f; |; . The restriction of the
true molar characters to the last three teeth on each side in each jaw, isan
important step towards the typical Mammalian dentition. Of the premo-
lars in the upper jaw, the last is the largest, and has a trihedral, pointed
crown. The true m olars have large crowns ; each supports six cusps, four
on the outer and two on the inner side. In the lower jaw, the long
crown of each true molar is divided into two parallel three sided prisms,
each of which is surmounted by three points ; each prism has one of its
angles turned outwards, and one of the faces inwards : the interspaces