254 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
not included the genera Pirnelodus and Cryptopterus in which only two or three 
species are at present known to possess degenerate air-bladders. 
In certain of the preceding genera the condition and relations of the air-bladder, 
and tlie character of the correlated skeletal modifications, are sufficiently well known 
to admit of comparison with those dissected by us. Such a comparison clearly proves 
that many of the former exhibit structural modifications of an extremely interesting 
character. On the other hand there are others of which scarcely anything is known 
beyond the bare fact that the bladder is more or less rudimentary and encapsuled by 
bone, and no data are available for comparison with other and better known types. 
Of the Hypostomatinse about which any reliable evidence exists as to the structure 
and relations of the air-bladder and Weberian mechanism, we may first of all refer 
to a series of genera which in future we shall term the Loricaroid types ; under this 
head we include Loricaria, Plecostomvs, and Callichthys. There are certain points with 
regard to these types which are still obscure, and more particularly the structure of 
that portion of the internal ear which is specially related to the Weberian medianism, 
but certain fiicts may, nevertheless, be mentioned as affording a basis for comparison, 
on the one hand, with such S. abnormales as we have been able to examine, and on 
the other, with such extremely modified forms as Acanthicus and Ilyjyophthalmiis. 
From Reissner’s observations on Loricaria, Plecostonius, and Callichthys, and 
from certain facts which our own investigations have enabled us to add, it may be 
inferred that in all these types an air-bladder is present in the form of two laterally 
situated sacs with structurally complete, but extremely thin walls, and connected by 
an intermediate narrow tubular portion, supported in Callichthys and Plecostomus, and 
possibly in Loricaria also, by an arch-shaped spicule of bone, similar to what Reissner 
has termed the “ processus bijugus ” in Acanthicus. So far as Plecostomus is concerned 
we could detect no trace of a ductus pneumaticus, and of the remaining genera our 
limited material was too badly preserved to admit of the point being determined. 
The air-sacs are completely enclosed within osseous capsules formed by the so-called 
dorsal and ventral laminm of the transverse processes of the fourth vertebra, 
precisely as described by Ramsay Wright in the case of Hypophthalmus, and the 
lateral or distal apertures of the two capsules are almost completely closed by the 
post-temporals. All the anterior vertebrae in front of the fifth, with, perhaps, the 
exception of the centrum of the first which may be absent, are indistinguishably 
anchylosed into a “complex” vertebra,"^ which in turn is firmly attached, if not 
actually anchylosed, to the posterior face of the skull. From the mode of articu- 
lation of the complex centrum with the basioccipital, it may be concluded, that, 
with the possible exception of Callichthys, nothing comparable to the singular for- 
ward dislocation of the coalesced anterior vertebrae, which is so characteristic of 
Hypophthalmus, has taken place in these Siluroids. But little is certainly known as 
to the structure of the auditory organ, but it is clear that the two sacculi are closely 
* From Soulnsen’s account (37) of riecuslumus, it is jwssiblc that even the tifth vertebra maybe included. 
