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TEETH OF THE VERTEBRATA. 387

in size exists between the inferior series and the corresponding teeth
above, those below being mnch the longer and more robust.
In their manner of implantation tliey cannot be said to be either
acrodont or pleurodont, but rather intermediate between the two. The
internal aspect of each jaw, \\liicli is remarkable for its breadth, is slightly-
bevelled internally, causing the outer edge to rise a little above the inner.
Nearer the outer than the inner edge of this bevelled surface are a num-
ber of low bony elexations, corresponding to the number of the teeth in
functional use, to the summits of which they are attached by ankylosis.
In some instances these elevations are so faintly indicated that the teeth
ajjpear to be soldered to th3 bevelled surface of the jaw directly. Just
internal to the basis of the functional teeth may be seen the successive
sets in different stages of development. In the recent state they are
covered by a fold of the gum, which likewise covers up the bases
of the functional teeth.
The form of the crown is that of a long, slender, sharp-pointed cone
curved inward and backward. The anterior surface of each tooth is
marked by a well-defined groove extending from the base to the apex.
It is somewhat deeper at the base than the summit, and is most distinct
in the teeth of the lower jaw. The intervals between the bases of the
teeth allow abundant room for the accommodation of poison-glands, the
secretion of which is conveyed down these grooves and thus injected
into the wound which the teeth inflict upon a prey.
Another group of curious and interesting reptiles is the Dinosauria,
which became extinct at the close of the Cretaceous Epoch. They are
of esjDCcial interest on account of their remarkable bird-like affinities,
and, according to the views of many authors, were the direct progenitors
of the struthious birds, or ostriches, emus, etc. They were mostly of
gigantic size, and some of them are remarkable for the great number
of teeth contained in the upper and lower jaws ; others, again, were
almost edentulous.
In the iguanodonts and hadrosaiirs, which are typical representatives
of the herbivorous division of this order, the crowns of the teeth are
somewhat expanded and are marked externally by vertical ridges, while
the internal portion is smooth and rounded. In Iguanodon the external
surface, to which the enamel is confined, is traversed by three vertical
ridges, separated by vertical grooves ; the anterior and posterior edges
were serrated, as in Iguana, before the crown was abraded by wear.
In the hadrosaurs there is but one vertical ridge, which is external in tlie
upper and internal in the lower teeth. The part which bears this ridge
is known as the enamel or cementum plate. Prof. Cope has recently
had the opportunity of satisfactorily determining the dental peculiarities
of this group of gigantic saurians, as exemplified by the genus Diclouiiis,
through the fortunate discovery of an almost complete skeleton bv Dr.
Russel Hill and the author in the Bad Lands of Dakota during the
summer of 1882.
According to Prof. Cope's description, there are in all two thousand
and seventy-two teeth. Of these, there were not more than two or three
hundred in use at one time, the others being arranged in successive rows
beneath, ready to take the place of the functional ones when they were