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TEETH OF THE VERTEBRATA. 385

the opposite set ; thus that tooth which is caniniform in the lower jaw
is the fourth, and bites in front of tlie corresponding tooth above. It is
received into a deep fossa in the upper jaw just internal to the alveolar
border at the point of junction of the maxillary Avith the preraaxilkuy
bone, or between the fifth and sixth teeth above. It not unfrequently
happens in old specimens that this fossa is converted into a foramen
leading to the external surface of the skull by the perforation of its
base. In such cases the point of the lower canine passes through the
upper jaw and appears upon the upper surface.
The only important distinction between the alligators and the croco-
diles consists in the fact that in the latter this fossa is open externally,
causing the tooth-line to be interrupted by a deep notch, whereas in the
latter it is intact.
Both the alligators and the more typical crocodilians are remarkable
for the breadth of the palate and the flatness of the muzzle, as well as
the alternate increase and decrease in the size of the teeth from before
backward ; but in the gavials the snout is very long, narrow, and almost
cylindrical ; the teeth, too, are more nearly equal and of more regular
...
proportions.
In the alligator the anterior teeth have conical crowns terminatino; in
^
.
sharp points, which are slightly recurved. The posterior ones have
more obtuse crowns, Avhich terminate below by a moderately well-defined
neck. In some species the anterior and posterior surfaces of the crowns
are produced into trenchant edges, which may be more or less serrated
;
in the alligator this is but faintly mai'ked.
The manner of succession is not different from that of the other lower-
vertebrates. If the root of a tooth in place be exposed, the suceessional
sets in various stages of development will be seen below and to the inside^
of it, arranged in the form of a nest of crucibles. This arrangement;
results by reason of the absorption of the inner wall of the root of the
tooth in place which the immediate successor causes. By this means the
point of its crown comes to occupy the pulp-cavity of the functional tooth.
In the order Lacertilia, which includes the lizards proper, a more
varied development of the dental organs is met with. As a general
rule, teeth are borne upon the pterygoid and palatine as well as upon
the maxillary, premaxillary, and mandibular bones. There are, how-
ever, some exceptions, one of which is afforded by our little " horned
toads" (Phrynosonut), in which the palatines and pterygoids are eden-
tulous. The teeth may be either " pleurodont " or " acrodont " in their
manner of implantation, but in certain extinct forms (e. g. 3Tosof(aurus)
l)oth conditions are to be observed. In the case of acrodontism the
bases of the teeth are soldered to the summits of slight elevations which
arise from the alveolar border of the jaMS. Pleurodontism, as has
already been mentioned, consists in the ankylosis of the base and outer
sides of the teeth to the outer wall and bottom of the dental groove.
Another variety of implantation, known as coelodontism, has been
described, in which the tooth has a permanent pulp-cavity, and is
attached to the outer wall, leaving the base free ; it should be men-
tioned that in pleurodonts the pulp-cavity is not permanent ; it
soon becomes obliterated, leaving the tooth solid.
Vol. I.—25