248 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
expanded proximally {c.g., Pimelodus sapo, P. pulcher, Eutropiichthys, Schilhichthys, 
&c.) ; or they may partake of the nature of transversely-arranged bony cylinders or 
funnels, open distally in the dry skeleton, closed in the fresh specimen by the lateral 
cutaneous areas, but otherwise with more or less complete osseous walls {e.g., Clarias, 
Saccohranchus, Callomystax, Cetopsis, &c.). The transverse processes of the fourth 
vertebra always form the dorsal and anterior walls of the recesses and sometimes, in 
addition, the posterior wall also ; rarely do they alone completely enclose tubular 
recesses (Cetopsis) : more frequently the posterior walls are formed by the transverse 
processes of the fifth vertebra (e.g., Callomystax, Clarias, &c.). Exceptionally, a 
slender, lateral bony outgrowth (ventral process) from each of the superficial ossifica- 
tions investing the sides of the complex centrum may become attached to the ventral 
wall of the corresponding air-sac (e.g., Bagarius and Glyptosternum), or, as in one or 
two instances (e.g., Clarias), the outgrowths may be strongly developed, and form 
no inconsiderable portion of the ventral walls of the two osseous funnels. The for- 
mation of a horseshoe-shaped recess by the transverse processes of the fourth 
vertebra in conjunction with similar but more extensive, plate-like, lateral out- 
growths from the superficial ossifications, which is open laterally and behind in the 
dry skeleton, occurs in one genus only (e.g., Ailia). When not completed ventrally 
by bone the recesses are usually closed by a tough fibrous membrane which also 
invests the corresponding walls of the contained air-sacs (e.g., Bagarius, Glypto- 
sternum, and many others). This membrane we regard as the result of a special 
development of the superficial connective tissue which in all normal Siluridte invests 
the external surface of the air-bladder, and is continuous anteriorly with the trans- 
verse membrane. In whatever way the osseous capsules or recesses may be formed 
they are almost always closed laterally by the external skin (lateral cutaneous areas), 
never, as in the more aberrant Loricaroid genera, by the post-temporals. 
In none of the genera examined by us was the encapsulation of the air-bladder by 
bone effected wholly or in part by the actual ossification of its proper wall, or tunica 
externa, although such instances do occur in other S. abnormales (e.g., Hypopliihalmus, 
and probably Acanthicus), and in several Cyprinidse (e.g., Colitis). In Callomystax 
it is probable that the ventral walls of the osseous funnels are in part formed by the 
extension of ossification from the decurved anterior margins of the transverse pro- 
cesses of the fourth vertebra into the fibrous sheet which often closes the ventral 
openings of the recesses, and it is also possible that in many other S. abnormales the 
exceptional width and downward curvature of the margins of these processes are due 
to the same cause. The lamellar outgrowths which form the ventral walls of the 
horseshoe-shaped recesses in Ailia are also probably due to an extension of ossifica- 
tion from the superficial bony deposit on the surface of the complex centrum into the 
same fibrous membrane ; and a similar origin may possibly be ascribed to the ventral 
processes of Clarias,* Glyptosternum, and BagaHus. 
As a rule the post -temporal retains its normal relations to the skull and to the 
* See also Sorensen (37). 
