where the lobes divide, and terminate each in a dilated cavity, or
medullary canal, about half way between the centre and the circumference
of the tooth. In like manner the inflected folds of cement
(b, l), converging from the circumference of the section, proceed
inwards in a straight line, half way or three fourths of the way to the
central pulp-cavity; the commencement of the lateral inflection of one
of the longest of these folds is shown at b. The inflected substance consists
of a very thin layer of cement. The calcigerous tubes radiate
according to the usual laws, and resemble in diameter, in the
width of their interspaces, in their secondary undulations, their
dichotomous bifurcations and small lateral branches, the same
tubes in the complex labyrinthic structure of the tusk, figured in
Plate 64 a . The exterior or first formed part of the basal portion of
the tooth resembles the above described apical part in the comparative
simplicity of the alternate folds of cement and processes of dentine
; but the central mass of the basal moiety of the tooth presents
t e labyrinthic disposition of these tissues : it may be said that the
simple exterior crust of the tooth incloses and is expanded by the
more complicated structure. The thickness of the exterior more
simple structure is nearly the same in the serial teeth and the large
tusks, so that the proportion of the central labyrinthic mass is greater
in the latter, and its structure is likewise more complicated. In the
serial teeth of the Lab. pachygnathus, the structure of the basal half
of the tooth resembles that of the same part in the Lab. leptognathus,
which is figured as seen in transverse section at PI. 63 b , fig. 2.
The alveoli, m both upper and lower jaws of the Lab.
pachygnathus are large, moderately deep, but complete. The outer
wall of the alveolar processes in the lower jaw of the Lab. pa-
chygnathus is not higher than the inner wall. In this structure,
t erefore, as well as in the division of the alveolar groove into
sockets, the Labyrinthodon manifests an aflinity to the Crocodilian
and Plenosaurianreptiles; but, on the other hand, a similarly complete
dental socket is present in certain Scomberoid and Sauroid
shes ; and, as in these fishes, the base of the tooth is anchylosed to
the socket in both the Labyrinthodon pachygnathus and Lab. leptognathus.
A still more striking Ichthyic character is manifested by the
Lab. pachygnathus in the continuation of the row of small teeth
anterior and external to the three larger teeth. For a double series of
teeth, thus occasioned, does not exist in the maxillary or preman-
dibular bones of any Batrachian or Saurian reptile; and, in the mammalia,
the only instance of such a disposition of the teeth is the
exceptional presence of the small incisors behind the dentes scalprarii
of the hares and rabbits; but the location of large tusks in front or
behind a row of smaller teeth is a character which has hitherto been
met with only in the jaws of fishes. This, therefore, must be
regarded as another of the Ichthyic characters which are retained in
the lowest forms of reptiles, and which is thus manifested in a new
way in the primeval Batrachians, whose dental characters and peculiarities
we have been endeavouring to interpret.
C-ffiClLIA.
90. In the extinct family of the Labyrinthodonts the Batrachian
type of organization was modified so as to lead directly from that order
to the highest forms of reptiles, viz: the loricate or crocodilian Sauria;
some of the existing edentulous genera of Bufonidee connect the
Batrachian with the Chelonian order; the family founded upon the
Linnsean genus Cmcilia forms the transition to the Ophidian reptiles.
The characters which retain the Csecilise in the Batrachian
order are generally known, and may be briefly enumerated as the
double occipital condyle, the biconcave vertebrae, the smooth mucous
integument with minute and concealed scales, and the branchial apertures
retained by the young some time after their birth. In the fixed
tympanic pedicle and the anchylosed symphysis of the lower jaw the
CaeciliEe are also far removed from the typical ophidian structures ; but
the teeth in their length, slenderness, sharp points, wide intervals and
diminished number begin to exhibit the characters of the dental system
of the serpent tribe (PI. 65, figs. 1 &2). They are implanted in a single
row upon the maxillary, intermaxillary and palatine bones, the upper
jaw being thus provided with two semi-elliptical and sub-concentric
series; there are also two rows of equal-sized teeth on the premandibu-
lar bones of the lower jaw, in certain species ; and the Ccecilia is the last
example, in the ascending survey which we have taken of the dental