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384 DENTAL ANATOMY.
internal to the median cusp is horizontal, and has its surface thrown
into conspicuous folds or wrinkles. The teeth of the lower jaw are
essentially like those of the upper. The vomerine teeth are small and
conical, and are disposed in two longitudinal rows.
In the typical genus (Diadcdes) there is a well-developed canine,
while in another member of the family {Helodedes) there are canines
and a double row of maxillary teeth upon either side in the dentigerous
surface of these bones.
In the other subdivision of this order—viz. Anomodontla—the den-
tition is reduced to large pointed, recurved tusks, which are lodged by
distinct sockets in the maxillary bones. The rest of the jaw is edentu-
lous, and was in all probability ensheathed in a corneous substance, as
in the existing turtles. Other extinct members of this subdivision,
notably Rhynchosaurus and Oudenodon, were entirely edentulous, and
in all probability were the ancestors of the turtles.
That division known as the Crocodi/ia includes the alligators, croco-
diles, gavials, etc., which are separated from the other Reptilia by a
number of important osteological characters, prominent among a\ hich is
the complete development of the bony roof of tlie mouth. Teeth are
supported by the premaxillary, maxillary, and dentary bones only, the
palatines and pterygoids having appi'oximately the same relations and
edentulous condition as in the mammalian skull. In no crocodilian so
far knoMU are the teeth ever ankylosed to the body of the bones upon
which they are borne, but, on the contrary, they are set in distinct
sockets disposed in a single row along the margins of the tooth-bearing
bones. In young specimens the alveoli are apt to be ill defined, more
especially toward the back part of the jaws, but as age advances the
bony partitions become more distinct. On account of each tooth having
a distinct alveolus, this division of the Eeptilia was formerly known as
the thecodonts, in contradistinction to the pleurodonts—a condition
already mentioned in connection with JlenojJoma—and acrodonts, pres-
ently to be described. •
A good example of the dentition of a crocodilian reptile is afforded
by the Mississippi alligator (AUigcdor J//.s.s/.ss?23p?V?Js/s), Avhich can be
found in almost any osteological collection in this country. In the
upper jaw there are from eighteen to twenty-two teetli upon either side,
of which five are usually set in each premaxillary and the remainder in
the maxillary bones. The most anterior of the premaxillary series is
the smallest, from which they gradually increase in size to the fourth,
which is nearly twice as large as any of the others ; the fifth is about
equal to the third. The first of the maxillary series is likewise the
smallest ; the three succeeding teeth gradually increase in size until the
third is rcached'(the ninth counting from the first tooth in the premax-
illary), which is known as the canine of the upper jaw. The eighth and
tenth are frequently as large as the canine. Behind, the teeth become
smaller, and are again enlarged in the vicinity of the sixteenth or sev-
enteenth from the first premaxillary tooth ; from this point they rapidly
diminish toward the posterior end of the tooth-line.
In the lower jaw the teeth are likewise of unequal proportion, but
those which are largest in the one series are opposed by the smallest of