ON THE ANATOMY OF FISHES. 
79 
also project outwards beyond it (fig. 3). In fact, the outer portion qf the post- 
temporal plate completes the circumference of a circular bony structure, of which the 
ventral and inner margins are furnished by the crescentic process. The latter also 
helps to slightly deepen the concavity of the posterior face of the plate by strengthening 
as it were its inner and ventral lips. Along the dorsal and inner margins of the pos- 
terior face of each plate there is a curved facet for articulation with a similarly shaped 
surface on the adjacent anterior edge of the root of the crescentic process (fig. 6, yi). 
The anterior face of the plate, which is greatly thickened towards the centre, is 
somewhat convex, and is also traversed by a groove continued dorsally into a deep 
tubular socket for the reception of the slender proximal extremity of the clavicle 
(fig. 6, cl.s.). Both the post-temporal plate and the stem of the bone are on each 
side firmly united to the crescentic extremity of the transverse process by ligamentous 
fibres, which, in turn, blend with the lateral margins of the transverse and aponeu- 
rotic membranes (vide p. 88). The union of each plate with the crescentic process is 
further strengthened by a strong ligament which passes between the concave margin 
of the latter and a slight depression near the inner edge of the posterior face of the 
former (fig. 3, Igt.). The post-temporal plates and the two crescentic processes 
combined form two slightly concave bony structures which are closely applied to the 
corresponding lateral portions of the anterior wall of the air-bladder (fig. 3). 
The clavicle (fig. 3, cl.), in addition to a slender head articulating with the post- 
temporal in the manner described, has a well-marked posterior process projecting 
backwards dorsad to the articulation of the pectoral fin, but considerably ventrad to 
the distal extremities of the modified transverse processes of the fourth and fifth 
vertebrae {cl.p.). 
The lateral line canal traverses a well-marked groove in the outer surface of the 
ascending process of the post-temporal as it passes forwards from the lateral surface 
of the trunk to the skull. 
The Auditory Organ. — We are not acquainted with any recorded observations as 
to the structure and relations of the various parts of the membranous labyrinth in 
Macrones, and the results of our own dissections are such that we have but little to 
add that is not merely confirmatory of the investigations of previous observers in 
other Siluroids, but as so very few species have been examined a brief description of 
the internal ear of Macrones may not be altogether superfluous. 
A comparison of figs. 8 and 9 will serve to indicate the relations of the different 
factors of the membranous labyrinth to one another and to the osseous chambers in 
which they severally lie. In fig. 9, the fibrous roof of the atria sinus imparis has 
been removed, and also the bony roof of the cavum sinus imparis and fovem sacculi, 
so as to show the mutual relations of the two sacculi, the sinus endolymphaticus, and 
the ductus endolymphaticus to one another and to the rest of the internal ear, as 
well as the relations of the sinus endolymphaticus to its cavum and to the two atria 
into which the latter opens posteriorly. 
