ON THE ANATOMY OF FTSHES. 
179 
aiiricularis,”* * * § while the ventral surface a little behind the middle of the capsule is 
traversed by a deep transverse gi’oove in which the first rih lies. Posteriorly the two 
capsules are somewhat contracted and terminate in widely separated free extremities. 
Certain apertures perforate the walls of each capsule. The ventral surface of the 
anterior and broader portion is fenestrated by several variously sized foramina. A 
small oval foramen, the “ apertura caj^sulae osseae interna inferior,” perfoi'ates the 
antero-internal inferior wall. A larger foramen, the ap. cap. oss. externa, traverses 
the anterior portion of the outer wall immediately below the postero-lateral cranial 
plates.t One or two channels also perforate the “ processus auricularis,” and, like all 
the preceding ones, communicate with the interior of the osseous capsule. From the 
apertura capsulae osseae interna inferior a slight groove extends on to the internal 
surface of the inner wall of the capsule in a direction upwards and forwards and finally 
terminates in an “apertura capsulae osseae interna superior through which the 
interior of the capsule communicates by means of a cavity in the anterior portion of 
the “ first ” vertebra with the hinder part of the cranial cavity. From the inner margin 
of each ap. cap. oss. int. inf a very delicate process of bone arises, which curves 
downwards and inwards and meets its fellow of the opposite side, thus forming a 
slender arch of bone. A somewhat stouter median spicule descends from the ventral 
surface of the body of the “ first ” vertebra and fuses with the apex of the arch 
formed by the two lateral processes. To this peculiar double arch depending from 
the under surface of the “ first ” vertebra Feissner gives the name of “ processus 
bijugus.” 
The air-bladder is constricted into two laterally situated oval air-sacs which occupy 
the cavities of the two osseous capsules. The walls of the sacs are everywhere in 
close contact with the inner surfaces of their investing capsules, but are only attached 
thereto by a few fibres which are said to transmit blood-vessels and possibly nerves. 
Although no connection between the two sacs could with certainty be determined 
Feissner regards it as extremely probable that such communication does exist, end 
believes that the tubular connecting duct is supported by the “ processus bijugus,” ^ 
and opens into each air-sac through the “ apertura capsulm ossem interna inferior.”|| 
* From the analogy of Colitis Rrissneu think.s tliat this prooe.ss may represent tlie distal end of a 
transverse process ; if this be so, it must belong to the fourth vertebra, but much more probably, how- 
ever, it is the post-temporal. 
t This may coi-respond to the distal aperture of the funnel-shaped transverse process in other 
abnormal Silurida?, or possibly to a very similar aperture which is found iii each of the outer walls of 
the osseous capsules of the Cyprinoid Cohif is and its allies. 
X An equivalent to this aperture is found in other Siluroids (e.f/., Callomystax) at the antero-internal 
angle of each bony capsule, where there is a foramen through which the tripus passes to its connection 
with the scaphium anteriorly. 
§ h’rom the analogy of Flecostomus we have no doubt that Reissner’s belief is correct. 
II There is one point in connection with the air-bladder in which Acanthictis is uni(jue among the 
Siluridne. Reissner describes the first ]>air of ribs as occupying well marked grooves on the venti'al 
2 .\ 2 
