groove. The bone is traversed its full length by th e preopercular latero-sensory canal, and lodge»
six sensory organs.
The HYOMANDIBULAR is an irregular bone, roughly triangular in general outline, one of
the angles of th e triangle being directed dorsally and bearing the articular heads of th e bone. Not
far from th e anterior edge of the triangle, and parallel with it, there is a slightly thickened portion,
which represents th e shank of th e bone. The hind edge of th e triangle is slightly convex, and bears
on its external surface a ta ll ridge which projects laterally and slightly anteriorly. The posterior
surface of this ridge is applied against, and firmly bound to th e antero-mesially presented anterior
surface of the dorsal limb of the preopercular, the dorsal end of th e la tte r bone reaching to the lower
edge of th e opercular process of the hyomandibular. This ta ll ridge along the hind edge of the hyo-
mandibular of Dactylopterus th u s corresponds to the ridge th a t extends longitudinally along th e
external surface of the shank of the bone in th e other fishes of th e group; b u t here, in D actylopterus,
although th e ridge begins on the external surface of the dorsal end of th e slightly indicated shank,
it extends postero-ventrally a t an angle to th e shank, and leaves, between itself and th e shank, a
wide intervening space which is spanned by a th in web of bone. The dorsal end of the ridge is
thickened to form a large articular head, which articulates with th e facet on th e antero-lateral corner
of th e squamosal, and is accordingly the regular posterior articular head of th e bone. Postero-mesial
to this articular head there is a stout and bluntly pointed process, directed antero-dorsally in th e line
of th e ridge on th e external surface of the bone. The postero-lateral surface of this process is slightly
concave, is grooved from its top to its base, and has a sliding articulation with th e a rticular surface on
the thickened posterior edge of th e articular facet on th e squamosal. From the anterior surface of the
base of this articular process a strong process is directed antero-ventro-mesially. I t expands a t its
distal end, and has a curved articular edge which is presented mesially and has a sliding articulation
with th e articular surface on th e postorbital edge of th e prootic immediately dorsal to th e trigemino-
facialis chamber. Fused with the lateral edge of the base of this process of th e hyomandibular, there
is a small process which arises from th e anterior surface of the ridge on th e external surface of the
bone, immediately ventral to th e regular posterior articular head. This small process is directed
anteriorly and has on its dorsal surface a slightly concave articular surface which articulates with th e
articular eminence on th e sphenotic. This small process accordingly represents the regular anterior
articular head of th e bone. Each of th e two regular articular heads of the hyomandibular of Dactylopterus
thus has a related, accessory articular head which has a sliding articulation with th e cranium,
this arrangement giving great solidity to th e joint.
On th e hind edge of th e bone there is a short opercular process.
On th e internal surface of th e bone, immediately ventral to the bases of th e two accessory
articular processes, there is a slight transverse ridge of bone, and on th e dorsal surface of this ridge
is th e internal opening of th e facialis canal. This canal traverses the hyomandibular and opens on
its external surface immediately posterior to th e slightly thickened portion th a t represents the shank
of th e bone. Immediately ventral to the transverse ridge, on th e internal surface of th e bone, a large
opening leads directly into th e facialis canal, and this opening transmits the ramus hyoideus, this
nerve thus never reaching th e external surface of the hyomandibular. The remainder of the facialis
canal transmits the ramus mandibularis lateralis certainly accompanied by communis fibers, and
possibly also b y certain general cutaneous fibers. The nerve so constituted is joined, as it reaches
th e outer surface of th e bone, by a communicating general cutaneous bundle which arises from th e
trigeminus ganglion and issues through th e trigeminus opening of the trigemino-facialis chamber,
this being th e second trigeminus bundle to join th e facialis, the other bundle issuing through
the facialis opening of th e trigemino-facialis chamber, joining th e nervus facialis internal to
the hyomandibular, and going largely, if not entirely, to the ramus hyoideus. The ramus
mandibularis, th u s formed, after giving off certain branches, passes downward and inward through
a large opening between th e hind edge of th e symplectic and th e anterior edge of th e web of bone
th a t spans the angle between th e two limbs of th e preopercular, this opening being the regular foramen
for th e ramus mandibularis externus. Certain fibers other th a n lateralis ones form p a rt of the nerve
th a t passes through th e foramen, b u t whether they are pa rtly communis or wholly general cutaneous
could not be determined in my material. I t is, however, certain th a t, in this fish, no important
bundle of communis fibers passes inward anterior to the symplectic, where the ramus mandibularis
internus should normally pass, and, in my material, I could not there find an opening th a t I could
certainly call a foramen and n ot an artifact. From the facialis canal, near its internal opening, a canal
leads backward in th e hyomandibular, as usual, opens on its external surface near its hind edge and
immediately ventral to the opercular process, and transmits a nerve containing lateralis and general
cutaneous fibers destined to innervate the dorsal two organs of the preopercular canal and tissues
on the outer surface of the opercular.
The ramus hyoideus facialis thus never reaches th e external surface of th e hyomandibular,
and i t accordingly here has relations to th a t bone th a t m ight be taken to indicate th a t it was in process
of cutting through the bone from its outer to its inner surface; b u t this is certainly not the case, the
condition being in some way related to the development of th a t tall ridge on th e external surface of
th e bone th a t gives support to th e preopercular. This ridge lies, in certain fishes (Scorpaena, Trigla etc.),
posterior to th e external opening of th e facialis canal through th e hyomandibular, while in others
(Scomber) it lies anterior to th a t opening. If, in one of these la tte r fishes, the ridge were to be pushed
backward into th e position, relative to th e shank of the bone, th a t it has in Dactylopterus, it would
probably pass over the ramus mandibularis, held in position b y its lower foramen, b u t would push
th e ramus hyoideus downward and backward onto what would be morphologically a p a rt of the
posterior surface of th e bone, though appearing as a p a rt of its internal surface. An intermediate
stage in such a process is shown in Gasterosteus, where, according to Swinnerton (’02, p. 544), „the
hyomandibular nerve foramen occupies th e same position as it did in th e la st stage, b u t externally
a t first sight it seems to have disappeared; in reality it has been carried to the ventral edge [of the
hyomandibular] by overgrowth of bone“.
The distal end of th a t slightly thickened portion of the hyomandibular th a t represents the
shank of th e bone is in synchondrosis with the usual interspace of cartilage, th a t cartilage being
bounded anteriorly by the ijietapterygoid, antero-ventrally by the symplectic, and posteriorly by
the web of bone th a t fills the angle between the two limbs of the preopercular. The interspace gives
articulation, on its internal surface, to a relatively long aiid important interhyal. Immediately
dorsal to this interspace of cartilage, the anterior edge of the hyomandibular is in contact with and
firmly bound to the dorso-posterior corner of the metapterygoid, and between the two bones and
th e interspace of cartilage there is a relatively large foramen which transmits the arteria hyoidea.
The SYMPLECTIC is broad and flat, and lies in the line, prolonged, of th e shank of th e hyomandibular.
Its ventral end overlaps the dorsal edge of th e quadrate, and lies in a slight depression
on th e internal surface of th a t bone. The anterior edge of the bone is bounded externally by the
Zoologica. Heft 57. 23