ON THE ANATOMY OF FISHES. 
137 
the development of antero-lateral or other caecal appendages in connection with the 
air-bladder. 
(e.) The more extensive attachment of the lateral portions of the tunica externa of 
the anterior wall of the bladder to the skeleton by the separation of their thick outer 
strata from the inner, and the dorsal insertion of the former into the transverse 
processes of the fourth vertebra. 
{/.) The existence of a distinct transverse membrane in addition to the skeletally 
attached outer stratum of the proper anterior wall of the bladder. 
(g.) The concentration of the inner stratum of longitudinal fibres in the median 
portion of the ventral wall of the anterior chamber into two stout, inwardly pro- 
jecting parallel ridges, and their extension into the anterior and posterior walls to the 
skeletal attachments of the anterior and posterior pillars respectively. 
{Ji.) The failure of the fibres of the inner stratum of the tunica externa in the 
antero-lateral and lateral walls of the anterior chamber to reach their usual insertions 
into the crescentic processes of the tripodes. 
(n).The suppression of the ascending and horizontal processes of the intercalarium 
and the reduction of that ossicle to a small nodule of bone imbedded in the inter- 
ossicular ligament. 
It may be remarked that as regards the features indicated in paragraphs h, c, e, h, 
and i, AHus more or less closely resembles Platystoma and the other normal Pimelo- 
djnse ; but the distinctness of the Arioid type is, nevertheless, sufficiently emphasized 
by the facts referred to under a, d, and f. 
In addition to Arius pidada, we have dissected the following Indian and Malayan 
species of the genus : — 
A. sagor. 
A. truncatus. 
A. ccelatus. 
A. venosus. 
A. utik. 
A. argyropleuron. 
A. maculatus. 
A. thalassinm. 
A. arioides. 
A. australis. 
Of the African species, we have only been able to examine one example, viz., 
Anus latiscutatus ; and of the South American representatives of this widely- 
distributed genus, a solitary example of A. assimilis. 
All these species very closely resemble one another and Arius pidada in the sti uc- 
ture of the air-bladder and the nature of the correlated skeletal modifications. 
Whatever variations do exist, appear to relate mainly to the relative thickness of the 
walls of the bladder, the greater or less number of the secondary transverse septa in 
the lateral compartments, or the relative size of the lateral chambers as compared 
with the anterior chamber. As a general rule, and as in most other normal Siluroids, 
the size of the air-bladder is approximately proportional to that of the Fish ; 
and in cases where it is otherwise, any diminution in its capacity takes place at the 
MDCCCXCTII. — B. 
T 
