980 
come elongated by the addition of successively 
calcified portions of their pulp to their basal 
or posterior extremities; the opposite end exerts 
a proportional pressure against the circum- 
ference of the foramen, and causing its ab- 
sorption begins to protrude. The tuberculate 
crown of the denticle is exposed about the 
time when its sides become anchylosed to those 
of the previously protruded row. Thus, from 
the close apposition of the protruding den- 
ticles, the whole of the outer parietes of their 
common alveolar cavity subjected to the sti- 
mulus of their pressure is finally removed, and 
is replaced by the pavement of mutually an- 
chylosed teeth (fig. 515, «). 
In the Diodons and Tetradons the structure 
of the teeth is equally peculiar, but of a very 
different character from what has been described 
above. In the former genus each jaw is fur- 
nished with a double compound tooth adapted 
to crush and bruise the food, the structure of 
which at once reminds the anatomist of the 
molar teeth of the elephant. Each tooth 
(fig. 517) consists of numerous laminz super- 
imposed upon each other, the upper ones 
being the oldest and most worn, while the 
lower ones are the largest and most recently 
formed. In consequence of this arrangement 
Fig. 517. 
Section of the lower jaw of the Globe-fish ( Diodon ), 
shewing the structure of its teeth. 
a, circumference of jaw; b, grinding surface of 
tooth within the mouth. (After Owen.) 
the exposed surface of the tooth that projects 
into the mouth (6) presents a grooved appearance, 
being formed of the edges of the contiguous 
lamella that are situated towards the upper 
part of the tooth. As these superior lamine 
are worn away, it is evident that they are con- 
tinually replaced by the advancement upwards 
of the inferior plates, so that the tooth is kept 
constantly efficient for service. The circum- 
ference of the jaw (a) is formed of super- 
imposed plates which grow from below in a 
precisely similar manner; but between these 
and the posterior laminated tooth a very diffe- 
rent structure is interposed, which is revealed 
by the microscope to consist of a series of 
narrow flattened denticles lying horizontally and 
at right angles to the anterior surface of the 
jaw. These denticles are developed in a cavity 
tween the outer and inner walls of the jaw, 
the floor of which is formed by a thin cribn- 
form osseous plate separating the cavity con- 
taining the teeth from the wide vascular canal 
which runs in the substance of the jaw. In 
PISCES. 
the Tetradons a somewhat similar structure of 
the dental organs is met with. 
The rostral teeth of the Saw-fishes, Pristis, 
are quite unique among the whole race of 
Fishes from the singular position which they 
occupy, as will be perceived by the a 
account of this strange apparatus ex 
from Professor Owen's elaborate treatise. 
_“ The maxillary teeth of the Saw-fish, which | 
is an active and predatory Shark, are, notwith- 
standing its habits, extremely small, simple, 
obtuse, and wholly inadequate to destroy and 
secure the prey requisite for its subsistence, 
But this seemingly imperfect armature of the” 
mouth is compensated for by the develope 
ment from the anterior ery of the head of 
very singular and formidable weapon provid 
with strong lateral teeth, and which from it 
resemblance to a saw has given rise to the ver 
nacular of ‘ Saw-fish,’ applied to the present 
species of Shark.” % 
In most of the Plagiostomes, but especiall 
in the group of Squaloids, a conical projection 
or cutwater is continued from the fore-part 
the head, and its framework is compesed © 
peculiar and superadded cartilages articulate 
to the anterior extremities of the frontal, 
and vomerine bones. These rostral cartilag 
in the Saw-fish (Pristis antiquorum) a 
blended into a horizontally flattened pla 
which is produced to a length equalling or 
third that of the entire fish: this process 
more completely ossified than any other p 
Fig. 518. 
= 
Rostrum of Saw-fish ( Pristis antiquorum ), she 
the marginal teeth. . 
« 
