main infraorbital canal, th e canal lying immediately beneath th e spinous ridge on th e dorsal surface
of the bone and lodging b u t one sense organ of th e line, innervated by th e ramus oticus. The
preopercular canal joins th e main infraorbital a t th e h ind end of th e section of canal enclosed in th e
pterotic, the pterotic thus containing no post-preopercular latero-sensory ossicle.
B A S I O C C I P I T A L .
The basioccipital has, on its dorsal, cerebral surface, two large longitudinal grooves, one on
either side, these grooves occupying th e entire dorsal surface of th e anterior two-thirds of th e bone,
excepting only th e narrow median and lateral portions th a t form the bounding walls of the grooves.
Each groove is continued backward, as a recess, into th e posterior th ird of th e bone, th e hind end
of each recess almost reaching the conical surface of th e median vertebra-like depression on th e hind
end of th e bone. The posterior half or two thirds of the uncovered portion of each of these grooves
is roofed b y a mesial, nearly horizontal process of the exoccipital of its side, th e grooves thus becoming
large and deep recesses in the cranial floor. Each groove lodges th e posterior portion of th e sacculus
of its side, the anterior portion of th e sacculus being lodged in th e saccular groove on th e cerebral
surface of th e prootic. On th e outer surface of the skull, the bounding wall of the basioccipital portion
of th e saccular groove forms the posterior portion of th e bulla acustica.
On th e dorsal surface of th e basioccipital, between the hind ends of th e saccular grooves,
there is a small median pit, sometimes separated into two parts by a th in transverse partition, this
partition inclining from th e mouth of the p it downward and backward toward its point. The pit,
whether simple or double, leads downward and backward almost to th e point of th e conical vertebra
like depression on the hind end of th e bone, approaching th a t point so closely th a t it sometimes
is separated from it by a thin layer, only, of bone. This small median p it is evidently th e homologue
of the cavum sinus imparis of Sagemehl’s descriptions of the Characinidae and Cyprinidae, b u t it
is, in Scorpaena, wholly uncovered, the hind edges of th e mesial processes of the exoccipitals reaching
only to its anterior edge and being restricted to roofing, on either side, a portion of th e corresponding
saccular groove. This la tte r groove, it may be noted, is relatively small in the Characinidae and
Cyprinidae, its anterior end passing b u t slightly beyond the hind edge of the prootic; and th e sacculi
of opposite sides are, in those fishes, connected by a canalis communicans, not found in Scorpaena.
Posterior to the cavum sinus imparis, th e narrow remaining portion of the dorsal surface
of th e basioccipital slopes downward and backward to the hind end of the bone, is slightly concave
and forms a small bounding portion of the foramen magnum. On either side of th e cavum sinus
imparis, there is a roughened surface which gives support to a corresponding portion of the ventral
edge of th e exoccipital, and immediately anterior to this surface, on either side of the anterior end
of th e cavum sinus imparis, th e basioccipital encloses a small nodule of cartilage, a remnant of the
chondrocranium, which lies between it and th e exoccipital. Anterior to this nodule of cartilage th e
thin lateral edge of th e basioccipital, on either side, is in sutural contact with th e ventral edge of
the exoccipital; while the median ridge of th e bone, which separates th e saccular grooves, gives
support to the mesial edges of the mesial processes of the exoccipitals, a small remnant of cartilage
there intervening.
On the ventral surface of the basioccipital there is a deep longitudinal groove, nearly circular
in transverse section. This groove lies between the bottoms of th e saccular grooves, tapers gradually
to its hind end, and forms the posterior portion of th e inyodome. I t opens on the ventral surface
of the basioccipital by a narrow median slit-like opening, which extends the full length of the groove
and forins th e posterior half of th e hypophysial fenestra. This fenestra, as already stated * is closed
ventrally, excepting in its posterior portion, by the underlying parasphenoid, the hind end of the
fenestra remaining uncovered, spreading somewhat, and so forming an opening which leads directly
into th e hind end of the myodome. On th e lateral surface of the hind end of the bone, immediately
dorsal to a horizontal plane through th e center of the vertebra-like depression on th a t end, there
isj on either side, a slight depression which gives insertion to the occipito-supraclavicular ligament.
This ligament of fishes is said by Sagemehl (’84 b, p. 49) to be a differentiation of the perimuscular
fasfcia of the adductor muscle of the shoulder girdle; b u t this adductor muscle, as described by
Sagemehl in the Characinidae, is not found in Scorpaena, and I doubt greatly this origin of the ligament.
The only muscle th a t a t all resembles SagemehPs descriptions of the adductor, is a flat, thin
muscle band th a t arises from th e hind edge of the skull and has its insertion on the occipito-supraclavicular
iigament near its outer end.
The basioccipital is bounded anteriorly, on either side, by the prootic, with which it is in
synchondrosis, and dorsally by the exoccipital with which it is partly in synchondrosis, and
partly in sutural contact. Its ventral surface is overlapped externally and largely covered by the
parasphenoid. Its hind end is wholly occupied by the deep conical vertebra-like depression.
E X O C C I P I T A L .
The exoccipital is an irregular bone which forms part of the lateral surface and part of the
posterior surface of the brain case, this latter portion having a medullary prolongation. The angular
edge th a t separates these two portions of the outer surface of the bone has, a t about the middle of
its length, a pronounced reentrant and usually well rounded angle; Anterior to this angle the edge
extends dorso-anteriorly and, reaching to the level of the floor of the hind end of the temporal fossa,
forms the ventral portion of the lateral bounding wall of the posterior opening of that fossa. Posterior
to the angle, the edge extends posteriorly and but slightly downward, and is thickened to form, at
its hind end, an articular head, which looks posteriorly and slightly ventro-mesially, articulates with
a process on the anterior edge of the first vertebra, and may be Called the condylar process of the
bone; The ventral Surface of this condylar process is roughened, and rests directly upon the lateral
portion of the dorSal surface of the hind end of the basioccipital, the two bones enclosing, a t the
anterior end of this suturating surface, the little nodule of cartilage already referred to when describing
the basioccipital. Anterior to this part of the exoccipital, the ventral edge of the bone is a
thin plate which rests upon and slightly overlaps externally the dorsal edge of the thin lateral portion
of the basioccipital, the two bones forming the lateral wall of the saccular recess of the cranial cavity.
That part of the exoccipital th a t forms part of the posterior surface of the skull is presented
dorso-posteriorly and is separated into two parts by a ventral continuation of the epiotic ridge, that
ridge forming the mesial boundary of the posterior opening of the temporal fossa. The medullary
prolongation of this part of the bone looks dorso-laterally, arches over the canal for the medulla
oblongata, and is in contact, in the median line, with its fellow of the opposite side. This contact
is by thin external and internal laminae of bone, the space between these laminae being filled with
cartilage. Extending obliquely across this part of the bone there is a marked thickening which begins
below and extends dorso-posteriorly, presenting a process-like appearance and strongly suggesting
a vertebral arch here fused with the exoccipital. The dorsal end of this process-like thickening forms
Zoalogica. He ft 57. o