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TEETH OF THE VERTEBRATA. 369

the branchial arches, Mere primarily used in connection with respira-
tion, so that any relations with the teeth which they may have subse-
quently acquired must be looked upon as a secondary modification.
This peculiarity, moreover, is of such wide application in the class
Pisces that a description of these parts cannot well be omitted in a con-
sideration of the accessory organs.
The hyo-branchial skeleton lies beneath the base of the cranium, and
is pretty well concealed in a side view by the opercular, hyo-mandibular,
and cpiadrate bones. It is connected with the rest of the skull at two
points—viz. by the articulation of the stylo-hyal bone with the hyo-
mandibular, and the other by means of loose connective tissue which
binds the upper portion of the branchial arches to the base of the cra-
nium. Its general structure will be best understood by describing it as
composed of a series of transverse bony arches placed one in front of the
other, rising up from the floor of the mouth and meeting in the median
line above.
The most anterior of these is the hyoid arch, which is formed by two
median basilar pieces upon either side, the basi-hyals. Passing from
within outward, we have first the cerato-hyah, to which are appended
the branchiostec/al rays. The next piece in the arch is the epi-hyal, fol-
lowing which is the stylo-hyal. This latter bone is a slender rod, and
serves to complete the connection of the hyoid arch with the hyo-man-
dibular bone. From the interval between the two most anterior basi-
hyals there projects a small bone forward which supports the tongue, and
is hence called the ento- or hyo-glossal. Projecting backward from the
inferior surface of these same basi-hyals is another piece, the uro-hyal.
Behind the hyoid, and similarly composed, are the five branchial
arches, of which the last two are somewhat modified. The three ante-
rior ones are made up of median basilar bones, the basi-branchihyals.
With these are articulated the hypo-branch ials upon the outside, after
which follow the cerato-branchial and epi-branchial pieces. In the
fourth branchial arch, counting from before backward, the hypo-
branchials are absent, and the uppermost segments are considerably
dilated and support teeth ; they are then known as the superior pharyn-
geal bones. The fifth arch is quite rudimentary, containing only the
cerato-branchial elements, which are generally much enlarged and bear
teeth ; these are the inferior pharyngeal bones.
The arrangement here described is found without substantial modi-
fication except as regards relative size and the degree of ossification of
the several parts in nearly all the sub-class Teleosfci. In the Elasmo-
branehii, however, the skull remains largely cartilaginous, and the hyo-
mandibular arch is always more or less imperfectly represented. The
maxillae and premaxill?e are likewise absent. In the chimeroid division
{Holocephall) neither the hyo-mandibular nor quadrate elements can be
made out, the mandible being attached directly to a broad triangular
cartilaginous lamella which stretches out from the sides of the base of
the skull, and whose anterior part bears the teeth of the upper jaw. It
will thus be readily understood that tliis cartilaginous plate, continuous
with the chondro-cranium, represents both the undifferentiated u]iper
portion of the hyo-mandibular and all of the palato-quadrate arches.
Vol. I.—24