extinct Dendrodus, as compared with that hitherto known to characterize
true fishes, is strikingly manifested both in the extent and
regularity of the radiating medullary processes, and more especially
in the straight course of the fine and close-set calcigerous tubes
which form the intervening dentine. Nevertheless I should most
probably have referred these teeth without hesitation to the class
of Fishes, if so close an analogy had not been traceable between their
singular dental structure, and that still more remarkable modification
which will be shown to characterize the extinct Batrachian genus
Labyrinthodon.
If, however, as seems most probable, the Dendrodus be a
true fish, we may speculate, from the similarity alluded to, upon its
having approached in its general organization, like the Lepidosiren,
most closely to the lower confines of the reptilian class : and as this
existing annectent genus is allied to the perennibranchiate Batrachians,
so the Dendrodus may have linked some extinct group of the class of
Fishes with the equally extinct family of Sauroid Batrachians which
we have termed Labyrinthodonts.
PART II.
DENTAL SYSTEM OF REPTILES.
C H A P T E R I.
GENERAL CHARACTERS OF TH E TEETH OF REPTILES.
67. T eeth, properly so called, do not exist in all reptiles; they are
absent in the whole order of Chelonia, in the Coluber scaber{ 1) among
the Ophidia, and in the toads among the Batrachia. In the latter edentulous
reptiles there is no compensating structure ; but in the Coluber
scaber, the inferior spinous processes of certain of the cervical vertebrae
are unusually prolonged, and penetrate the coats of the oesophagus ;
their extremities, which are thus introduced into the alimentary canal,
are coated with a layer of hard dentine, and form substitutes for
the teeth, which, if not always entirely absent, are merely rudi-
mental in the ordinary situations in the mouth. (2)
In the tortoises and turtles the jaws are covered, as is well known,
by a sheath of horn, which in some species is of considerable thickness
and very dense ; its working surface is trenchant in the carnivorous
species, but variously sculptured and adapted for both cutting and
bruising in the vegetable feeders.
The development of the continuous horny maxillary sheath commences,
as in the parrot-tribe, from a series of distinct papillæ, which
sink into alveolar cavities, regularly arranged (in Trionyx) along the
margins of the upper and lower jaw-bones : these alveoli are indicated
by the persistence of vascular canals long after the originally separate
tooth-like cones have become confluent and the horny sheath completed.
The teeth of the dentigerous Saurian, Ophidian and Batrachian
reptiles, are, for the most part simple and adapted for seizing and
(1) Hence called Anodon typus by Dr. Smith.
(2) Dr. Jourdan, in Cuvier, Leçons d’Anatomie Comparée, Ed. 1835, tom. I, p 340,
tom. 4, p. 617.
N 2