ON THE ANATOMY OF FISHES. 
75 
the vertebral column presents the appearance of a thin, laterally compressed, median 
bony keel, when viewed from the ventral surface. 
The Cranium . — In common with all Siluroid Fishes the skull in Macrones exhibits 
not a few features wherein it differs considerably from the normal Teleostean type, 
but we shall here refer in detail only to such of its structural peculiarities as are more 
or less closely correlated with certain characteristic modifications of the membranous 
labyrinth of the internal ear. 
The dorsal surface of the hinder part of the skull (figs. 1 and 4) is formed by the 
supraoccipital (s.o.), the epiotic (ep.o.), the pterotic {pt.o.), and the sphenotic {sp.o.) 
bones. In the absence of distinct parietal bones the supraoccipital is exceptionally 
large ; anteriorly it is deeply cleft by the posterior cranial fontanelle, while pos- 
teriorly it is produced into a spine-like process which extends backwards dorsad to 
the confluent spines of the third and fourth vertebrae as far as the first interspinous 
bone of the dorsal fin (fig. 1, so.'). Laterally, the supraoccipital articulates with the 
epiotics, pterotics, and sphenotics ; anteriorly with the frontals, and below with the 
exoccipitals. The posterior face of the bone, immediately over the foramen magnum, 
is traversed by a median ridge which, superiorly, is confluent with the horizontally 
flattened root of the supraoccipital spine (fig. 6), and on each side of the ridge, just 
beneath the root of the spine, there is a small foramen for the transmission of the 
lateral branch of the fifth cranial nerve (fig. 6, V^.). 
Each exoccipital (figs. 1 to 7, eo.) consists of a thin, laterally compressed neural 
plate, which is situated at the side of the foramen magnum, and meets the supra- 
occipital above, and, in addition, is prolonged forwards into the cranial cavity as a 
vertical plate of bone, the free anterior edge of which forms the posterior boundary of 
a large aperture leading from the general cranial cavity into a spacious lateral recess 
in which the utriculus and its semicircular canals are lodged (figs. 2 and 5, eo.'). From 
the outer surface of its neural plate each exoccipital gives off a posterior plate which 
extends outwards at right angles to the foregoing on to the hinder face of the skull 
and there articulates dorsally with the supraoccipital and the epiotic (fig. 6, eo.^). 
The posterior plate eventually extends round the postero-lateral angle of the cranium 
on to the lateral surface, where it occupies the place normally taken by the opisthotic 
in most other Teleostean Fishes, meeting the prootic in front, the lateral margin of 
the basioccipital below and the pterotic and epiotic bones above (figs. 3 to 5, eo.^). 
The opisthotic plate of the exoccipital is extremely thin, and, in the complete absence 
of a true opisthotic element, is in relation internally with the inferior part of the arch 
of the posterior vertical semicircular canal. A small foramen perforates the anterior 
margin of the plate, near its junction with the prootic, and transmits the glosso- 
pharyngeal nerve (fig. 4, IX.), and somewhat more posteriorly there is a much larger 
aperture for the exit of the vagus (X.). The hypoglossal or first spinal nerve emerges 
from the cranial cavity through a small foramen in the posterior plate, situated in the 
lower part of the angle which the latter makes with the neural plate (fig. 4, sp.n.'). 
L 2 
