the posterior portion of the dorso-mesial edge of the medullary prolongation; and the posterior portion
of the ventral edge of the spina occipitalis is wedged in between the ends of the processes of opposite
sides. Anterior to these process-like portions of the exoccipitals the ventral edge of the spina occipitalis
rests on the external surface of the suturating edges of the exoccipitals. In the hind edge of the
medullary prolongation of the exoccipital, there is a large semicircular notch, but it is closed with
fibrous tissue and does not give passage to any structure.
From the internal surface of th a t part of the exoccipital th a t lies immediately dorsal to the
saccular groove, a stout plate-like process projects mesially and slightly downward, and expanding
slightly a t its mesial end is in synchondrosis, in the middle line of the head, both with its fellow of
the opposite side and with the dorsal edge of th a t thin median ridge of the basioccipital th a t separates
the saccular grooves; a remnant of cartilage intervening between the three bones. The process forms
the roof of a considerable^ portion of the saccular recess, but does not roof any portion of the cavum
sinus imparis, as Sagemehl says th a t it does in the Characinidae and Cyprinidae, the hind edge of
the process only reaching to the anterior edge of th a t pit. Immediately dorsal to the base, or line
of origin, of this mesial process, the exoccipital is perforated by two foramina, one of which lies near
the anterior edge of the process, and the other near its posterior edge. The anterior foramen opens
on the lateral surface of the bone and transmits the nervus vagus, the other opening on the base
of the condylar process of the bone, being sometimes double, and transmitting the occipital nerves.
Slighty anterior to the vagus foramen the bone is perforated by the glossopharyngeus foramen.
On the internal surface of the bone, immediately dorso-anterior to the vagus foramen, there is
a recess which lodges the ampulla of the posterior semicircular canal. From this recess two canals
start, one running upward and enclosing a portion of the membranous posterior semicircular canal,
while the other runs latero-anteriorly and encloses part of the external canal. The canal for the
external semicircular canal lies antero-intemal to the angular edge between the lateral and
posterior surfaces of the bone, the canal for the posterior canal lying in the ridge th a t forms
a ventral prolongation of the epiotic ridge.
The exoccipital is bounded ventrally by the basioccipital, anteriorly by the prootic, dorso-
laterally by the pterotic, and dorsally by the epiotic and supraoccipital, from all of which bones
it is largely separated by lines of cartilage. The opisthotic overlaps externally the dorsal portion
of the lateral surface of the bone, fitting into a depressed region on th a t surface.
O P I S T H O T I C .
The opisthotic (intercalar) is a small plate-like bone, quite unquestionably of purely ectosteal
origin, which forms the middle portion of the postero-lateral edge of the skull; there overlapping
externally the adjoining edges of the pterotic and exoccipital, and extending forward to, or even
slightly overlapping the hind edge of the prootic. Its hind edge is thickened and projects backward
beyond the pterotic and exoccipital, there forming part of the lateral wall of the temporal fossa.
A small eminence on this edge gives support, and is bound by ligamentous tissue to the ventral end
of the opisthotic process of the suprascapular.
. E P I O T I C .
The epiotic (exoccipitale) is a somewhat pyramidal bone which caps the dorso-mesial corner
of the hind end of the temporal fossa and has dorsal, posterior, lateral and cerebral surfaces. A portion
of its dorsal surface gives support to the overlying parieto-extrascapular, the remaining part of that
surface forming the lateral portion of the floor of thesupratemporal pocket; this latter part of the bone,
as already explained, lying on and appearing as a part of the posterior surface of the skull. The
posterior surface of the bone forms part of the posterior surface of the skull; its lateral surface forming
the mesial wall of the temporal fossa. The angle between these two latter surfaces forms a strong,
epiotic ridge, which lies in a nearly vertical position near the middle of the corresponding half of
the posterior surface of the skull, forms the mesial boundary of the posterior opening of the temporal
fossa, and, with its ventral prolongation on the posterior surface of the exoccipital, marks the
course of the posterior semicircular canal. The summit of the bone is directed dorso-postero-laterally,
and from it a flat, and often sharp and slender process arises, directed postero-laterally and slightly
ventrally. On the dorsal surface of this process, between it and the overhanging posterior portion
of the parieto-extrascapular, the epiotic process of the suprascapular rests, the two processes roofing
the hind end of the temporal fossa. The cerebral surface of the bone is wholly occupied by a deep
conical pit which forms part of the labyrinth recess and lodges the dorsal portion of the posterior
semicircular canal, th a t canal piercing the ventro-posterior wall of the recess and from there running
downward through the bone, internal to the epiotic ridge.
The epiotic is bounded antero-laterally by the pterotic, ventrally by the exoccipital, and
mesially by the supraoccipital, with all of which bones it is in synchondrosis.
S U P R A O C C I P I T A L .
The supraoccipital forms part of the dorsal and part of the posterior surface of the skull,
these two parts, or limbs of the bone lying a t an obtuse angle to each other. From the postero-ventral
limb of the bone, and extending its full length, a large thin spina occipitalis projects directly backward,
the ventral edge of the spina lying partly upon, and partly being enclosed between, the adjoining
dorso-mesial edges of the medullary prolongations of the exoccipitals. The dorsal limb of
the bone projects forward between the lateral fontanelles in the roof of the primordial cranium, and
is overlapped anteriorly by the frontals, and laterally, on either side, by the parieto-extrascapular.
Between these three overlapping bones, a small and variable portion of the supraoccipital is exposed
on the dorsal surface of the skull and forms part of the floor of the subquadrangular groove on the
vertex. A strong transverse ridge, projecting dorso-posteriorly, crosses the hind edge of the dorsal
surface of the bone, and against the anterior surface of this ridge the mesial processes of the parieto-
extrascapulars rest. The postero-ventral limb of the bone is crossed, near its dorsal end and on either
side of the spina occipitalis, by a more or less prominent transverse ridge, and th a t part of this limb
of the bone th a t lies dorsal to this ridge forms, on either side, the mesial portion of the supratemp-
oral pocket. The cerebral surface of the bone forms median portions of the roof and hind wall of
the cranial cavity. The bone is bounded ventrally by the exoccipitals, and laterally, on either side,
by the epiotic, with all of which bones it is in synchondrosis. Anteriorly the bone is edged with a band
of cartilage, anterior to which the lateral fontanelles of opposite sides are confluent; the band of
cartilage representing all that is found, in this fish, of the large postepiphysial cartilage of Scomber.
C R A N I A L CAV I T Y .
The cranial cavity extends forward to about the middle of the orbit. The mid-longitudinal
line of the floor of the cavity slopes downward and backward from its anterior end to the anterior