ON THE ANATOMY OF FISHES. 
101 
vertebra, the two structures together forming on each side a concave bony surface, 
which is closely applied to the adjacent antero-lateral wall of the bladder. The latter 
organ is proportionally larger than in other species of the genus, but the cavities of 
its lateral compartments are much more broken up and sub-divided through the 
greater development of secondary transvei’se septa and their buttress-like ridges. 
The modifications initiated in M. Iloevenii are carried to an extreme degree in M. 
aor. In this species each post-temporal plate instead of being merely concave on its 
posterior face, as in M. Iloevenii, becomes curiously bullate and goblet-shaped, witn 
the mouth directed backwards and the much contracted stem continuous with the 
inferior limb (fig. 25, pt.p.). Although the mouth of the bulla is directed backwards, 
its cavity, which gradually contracts, seems to extend inwards as an excavation in the 
inferior limb, nearly as far as the articulation of the latter with the basioccipital. Ihe 
crescentic distal extremity of the modified transverse process encircles and strengthens 
the ventral and inner lips of the mouth of the bulla, and is firmly attached thereto by 
ligament. From the antero-lateral angles of the anterior chamber of the air-bladder 
two csecal diverticula are given off, which are just sufficiently large to extend into and 
fill the cavities of the two bullae. The diverticula are composed of both tunics of the 
bladder, but the tunica externa is extremely thin. With the exception of the antero- 
lateral caeca the walls of the air-bladder are much thicker than is usual in Macrones. 
The ventro-lateral margins of the anterior chamber are curiously thickened internally 
by strong rib-like aggregations of the vertical fibres of the inner stratum of the tunica 
externa, which separate a series of small slit -like sacculi. The cavities of the lateral 
compartments are partially obliterated by the excessive development of the secondary 
transverse septa and their buttress-like bundles of fibres, as well as by the growth of 
vertically disposed fibrous columns extending between their dorsal and ventral walls. 
Marginal bundles of fibres are also developed in connection with the lateral walls of 
these compartments, and extend inwards in the form of incomplete septa ; the latter, 
with the smaller buttresses which they give off, to a large extent obliterate the lateral 
passages by which the compartments normally communicate in front with the anterior 
chamber. The lateral walls of the anterior chamber are not in direct contact with the 
superficial skin, the two structures being more or less completely separated by the 
anterior lobes of the mesonephros, but in general structure, and in the relations and 
attachments of its fibres to fixed or moveable portions of the skeleton, this section of 
the air-bladder presents no special peculiarities. The Weberian ossicles are almost 
precisely similar to those of M. nemuriis. The ascending process of the intercalarium, 
relatively to the horizontal process, is rather stouter than usual, the latter being 
extremely thin and slender, and, moreover, is forked at its outer extremity where 
imbedded in the interossicular ligament. As in several other Siluroids, the posterior 
wall of the atrial cavities is strengthened by a small median process of bone, apparently 
derived from the posterior and dorsal margin of the basioccipital, but more probably 
due to the ossification of the median portion of the fibrous wall Itself. The process is 
