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TEETH OF THE VERTEBRATA. 377

lower or working face presents internally a considerable flat pitted surface
reaching the entire length of the tooth, whose plane is directed outward
and a little upward. It is slightly broader in front than behind. The
outer border is indented by five wide vertical grooves,.forming six verti-
cally convex lamelliform projections, M'hich encroach sonie\\'hat upon the
flat surface internal to them as well-defined parallel ridges ; the anterior
one passes entirely across the face of the tooth, or, rather, skirts its anterior
margin, and becomes continuous with the slightly elevated internal bor-
der in this situation. The grooves are deeper in front than behind, leav-
ing the anterior projections the most pronounced, while the kist one is
scarcely perceptible. The points of the projections describe a gentle
curve from before backward, the convexity being outward.
The two teeth of the lower jaw are very similar to those of the
upper, but are somewhat narrower, owing to a decrease in width of the
flat pitted surflice. As already stated, they are ankylosed to two plates
of bone which cover the inner and half of the lower surface of INIeckel's
cartilage or the central axis of the lower jaw. It will thus be seen that
each ramus of the mandible consists of an inner and an outer bony plate
enclosino; the central cart ilag-inous axis, which meet in the median line
below. To these is added a third piece at the symphysis on its lower
surface, so that the bony part of the jaw—which in all probability corre-
sponds with the single dentary piece in other fishes—is made up of
three, one of which bears teeth. This fact is of great morphological
significance, and will be referred to again when we come to discuss the
attachment of the teeth.
In the two allied genera Protoptenis and Lepidosiren the teeth are
very similar to those of Ceratodus, but in numerous extinct forms refer-
red to the dipnoans a considerable amount of variety exists. As regards
the development of the teeth in this group, very little is known, and
until this has been studied it will be impossible to say whether the den-
tal j)lates are moulded upon a single papilla or represent the combined
calcification of several. Judging from their complexity, the latter
would seem to be the case.
The teeth of the remaining ganoids are of the ordinary conical form
which prevails to so great an extent in the Teleostei. In number, posi-
tion, replacement, etc. they likewise agree so closely with the average
teleo.st dentition that it is unnecessary to make any further mention of
them.
Among teleosts, however, there are several well-marked modifications
in the dental armature which deserve to be noticed. One of these is
presented by the Plectognathl (plaited jaw), of which the "trigger-fish"
(Balistes vetuhis) and the " swell toad " (Diodon geometricus) represent
the extremes in dentition. The teeth of the mouth of Bolides are
twenty-two in number, of which fourteen belong to the upper and
eight to the lower jaw. Those of the superior series are disposed in
two rows, one placed immediately behind the other, and both are lodged
in the premaxillary bones. The most anterior of these rows contains
eight teeth, while in the posterior one there are only six. In the front
row the mesial pair are the largest, of a subtrihedral form, tapering
gradually to an obtuse point. Those upon either side decrease regu-