the hyomandibular in the angle between the opercular arm and the shank of the bone, and the other
close to the hind edge of the web of bone th a t fills the space between the same two arms of the bone.
This second and branching canal transmits the two branches of a nerve th a t supplies the two dorsal
latero-sensory organs of the preopercular canal.
The ramus hyoideus separates from the truncus hyoideo-mandibularis as that nerve reaches
the external surface of the hyomandibular, passes downward and backward through a small passage
between the hind edge of the hyomandibular and the anterior edge of the preopercular, and so
reaches the internal surface of the latter bone.
S YMP L E C T I C .
The symplectic is a slender curved bone, the dorsal two-fifths of which lie along the hind edge
of the cartilage th a t separates the metapterygoid and quadrate, while the ventral three-fifths lie
in the symplectic groove on the internal surface of the quadrate. The ventral end of the bone is tipped
with cartilage. Its dorsal end is bounded by the interspace of cartilage th a t lies between itself and
the hyomandibular. This interspace of cartilage is in close contact with the hind edge of the palato-
quadrate cartilage, but is not continuous with th a t cartilage. A part of the hind edge of the interspace
of cartilage is overlapped externally by a thin web of bone on the anterior edge of the preopercular,
near the middle of its length, and the hind edge of this part of the cartilage bears the articular
facet for the proximal end of the interhyal. Between the hind edge of the dorsal portion of the symplectic,
anteriorly, and the anterior edges of the preopercular and the posterior process of the quadrate
posteriorly, there is a long oval, space which transmits the ramus mandibularis externus facialis
and the arteria hyoidea. Along the anterior edge of the symplectic, between i t and the hind edge
of the dorsal portion of the body of the quadrate, there is a small opening which transmits the ramus
mandibularis internus facialis.
P R E O P E R C U L A R .
The preopercular is a curved bone, traversed its full length by the preopercular latero-sensory
canal. I t has, on its hind edge, five so-called spines, the two ventral ones being blunt or pointed
eminences, rather than spines. The dorsal spine is by far the longest and is always double, a small
spine, almost completely fused with it, arising on the external surface of its base. At the ventral
edge of the base of this small spine, and hence on the external surface of the base of the large spine,
there is the opening of a primary latero-sensory tube; and similar openings are found a t the ventral
edges of each of the three next distal spines. The fifth spine lies a t the distal end of the bone, is an
eminence rather than a spine, and immediately distal to it there is a primary tube which arises from
the sensory canal as it passes from the preopercular into the mandible. The spines, thus here, as
on the lachrymal, have definite relations to the primary tubes of the latero-sensory system; but
there is not a spine for every tube, for dorsal to the most dorsal spine there is, in the preopercular,
still another opening of the latero-sensory canal, but without related spine. The bone lodges six
latero-sensory organs, one between each two adjoining tubes.
At the middle of the anterior edge of the preopercular, spanning the hollow of the curve of
the bone, there is a thin web of bone which bears, on its internal surface, a small cup-like depresión,
this depression receiving the lateral surface of the proximal articular head of the interhyal and to
th a t extent forming part of the articular cup for th a t element. Lateral to this web of bone, or
slightly dorsal to it, on the raised and ridge-like external surface of the preopercular, there
is a shallow groove which marks the line of insertion of the hind end of the third bone of the
infraorbital chain.
O P E R C U L A R BON E S .
The three opercular bones have the shapes shown in the figures. On the external surface of
the opercular, three pronounced ridges radiate from the articular facet of the bone. One of these
ridges forms the ventral edge of the bone, which edge is presented anteriorly and but slightly vent-
rally; the other two ridges lying on the dorsal portion of the bone and both of them terminating
in free spines. The internal surface of the bone, dorsal to the dorsal one of the three spinous ridges,
is depressed, and in this depression the adductor operculi has its insertion; the thin and almost membranous
levator operculi being inserted along the dorsal edge of the bone. The deep indentation in
the hind edge of the bone, between the two dorsal spines, corresponds to the indentation, without
related spines, in the hind edge of the bone of Scomber.
The angular-shaped subopercular overlaps internally, and embraces the ventral corner of
the opercular, extending upward one half to two-thirds the length of its anterior edge, but along
its entire posterior edge; usually projecting upward slightly beyond the dorsal edge of the bone.
I t is a thin, flat bone, its long dorso-posterior arm, in particular, being so thin that it is flexible and
easily torn.
The dorso-posterior edge of the interoperculum is slightly concave, and lies in a nearly horizontal
position. The posterior comer of this edge slightly overlaps externally, and is strongly bound by tissue
to the ventral corner of the subopercular, while the anterior corner lies external to the interhyal,
and is strongly bound to it by tissue. The lateral surface of the ventral half of the interhyal here
fits into a large but shallow depression on the internal surface of the interopercular, this depression
having a raised dorsal edge which gives it the appearance of an articular facet. The interopercular
is thus here related to the interhyal somewhat as the branchiostegal rays are to the ceratohyal,
suggesting it being such a ray. Between its concave postero-dorsal edge and the anterior edge of the
subopercular there is a large triangular space, spanned by a sheet of tough fibrous tissue which
connects the bones. The ventral end of the interopercular is directed antero-ventrally and gives
attachment to a short strong ligament which has its origin on the hind end of the angular.
In Phractolaemus Ansorgii, Ridewood says (’05 a, p. 279) th a t th e interopercular is traversed
b y a portion of the preopercular latero-sensory canal, adding th a t this is the only instance of the kind
known to him. If the section of canal enclosed in th e bone lodges a latero-sensory organ, the bone
can not be a simple interopercular.
MA N D I B L E .
The mandible has, on its outer surface, a large rounded longitudinal ridge which extends
irom the ventral edge of the articular facet for the quadrate forward across the articular and then
across the dentary, nearly to the anterior end of the latter bone. On the inner surface of the articular
and dentary there is a corresponding hollow, which lodges, in its ventral portion, the rod-like
Meckel’s cartilage. Immediately ventral to the ridge, the mandibular latero-sensory canal traverses
the dentary and articular, entering the dentary near its anterior end and leaving the articular at
the base of the process th a t forms the posterior half of the articular facet for the quadrate. The