form as yet discovered in the class of reptiles. In no reptile does
the base of the tooth ever branch into fangs.
71. Attachment.—As a general rule, the teeth of reptiles are anchy-
losed to the bone which supports them. When they continue distinct,
they may be lodged either in a continuous groove, as in the Ichthyosaur,
or in separate sockets, as in the Plesiosaur and Crocodilians.
The base of the tooth is anchylosed to the walls of a moderately deep
socket in the extinct Megalosaur and Thecodon. In the Labyrintho-
dons and Csecilise among the Batrachians; in most Ophidians; and
in the Geckos, Agamians, and Varanians, the base of the tooth is
imbedded in a shallow socket and confluent therewith(l). In the
Scincoidians, Safe-guards (Tejus), in most Iguanians, in the Chameleons
and most other Lacertian reptiles, the tooth is anchylosed
by an oblique surface extending from the base more or less upon the
outer side of the crown to- an external alveolar plate of bone, as
shown in Plate 67 ; the inner alveolar plate not being developed. In the
frogs, the teeth are similarly hut less firmly attached to an external
parapet of bone. The lizards which have their teeth thus attached
to the side of the jaw are termed ‘ Pleurodonts.’ In a few Iguanians,
as the Istiures, the teeth appear to he soldered to the margins of the
jaws, these have been termed | Acrodonts.’ In some extinct Lacer-
tians, as the Mosasaur and Leiodon, the tooth is fixed upon a raised
conical process of bone, as shown in Plate 68, fig. 1 and Plate 72,
fig. 2.T
hese modifications of the mode of attachment of the teeth of reptiles
are closely adapted to the destined application of those instruments
and to the habits of the species; we may likewise perceive that they ofler
a close analogy to some of the transitory conditions of the human teeth.
There is a period, for example, (2) when the primitive dental papillae
are not defended by either an outer or an inner alveolar
process, any more than their gigantic calcified analogues in the
extinct Mosasaur. There is another stage(3) in which the groove
(1) PI. 63 a, fig. 8. a a, sockets of the anchylosed teeth in Varanus var 'ms.
(2) At the sixth month, see Mr. Goodsir, On the development of the Human Teeth..—Edinburgh
Medical and Surgical Journal, No. 138.
(3) At the seventh or eighth week.—Ibid.
containing the dental germs is defended by a single external cartilaginous
alveolar ridge : this condition is permanently typified in most
existing lizards. Next there is developed an internal alveolar plate,
and the sacs and pulps of the teeth sink into a deep but continuous
groove, in which traces of transverse partitions soon make their
appearance : in the ancient Ichthyosaur the relation of the jaws to
the teeth never advanced beyond this stage. Finally, the dental
groove is divided by complete partitions, (1) and a separate socket is
formed for each tooth, and this stage of development is attained in
the highest organized reptiles, as in the crocodile.
72. Substance.—This may be four-fold, and a single tooth may
be composed of dentine, cement, enamel and bone ; but the dentine
and cement are present in the teeth of all reptiles.
In the Batrachian and Ophidian reptiles a thin layer of cement
invests the central body of dentine, and as usual, follows any inflections
or sinuosities that may characterise the dentine. Besides the
outer coat of cement, which is thickest at the base of the teeth, a
generally thin coat of enamel defends the crown of the tooth in most
Saurians, and the last remains of the pulp are not unfrequently converted
into a coarse bone, both in the teeth which are anchylosed to
the jaw, and in some teeth, as those of the Ichthyosaur, which
remain free. The only modification of the dentine, which could at
all entitle it to be regarded in the light of a new or distinct substance,
is that which is peculiar, in the present class, to the teeth of the Iguanodon,
and which will be described in the following section.
73. Structure.—The varieties of dental structure are few in the reptiles
as compared with either fishes or mammals, and its most complicated
condition arises from the interblending of the dentinal and other
substances rather than from modifications of the tissues themselves.
In the teeth of most reptiles the intimate structure of the dentine
corresponds with that which has been described as its fourth type or
modification in the teeth of fishes, and which is the prevailing structure
of mammalian dentine, viz : the radiation of a system of minute cal-
cigerous tubes from a single pulp-cavity, at right angles to the external
surface of the tooth. The most essential modification of this structure
(O At the sixth month.—Edinburgh Medical and Surgical Journal, No. 138.