136 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
anterior wall has much the same value both in Arius and Platystoma, and in each 
case is wholly or partially a portion of the tunica externa ; and further, that the 
transverse membrane as a separable and distinct structure is entirely wanting in 
Platystoma — a fact which may possibly be associated with the extensive insertion of 
the great compressor muscles into the walls of the bladder. We have elsewhere, in 
our description of Macrones, given reasons for believing that even the transverse 
membrane is merely a special portion of the superficial coat of the air-bladder, which, 
for physiological purposes, has acquired certain fixed skeletal attachments, and with a 
similar object it is readily conceivable that, in addition, even a stratum of fibres 
belonging to the tunica externa of the anterior wall may also acquire a similar con- 
nection with rigid portions of the axial skeleton, such, in fact, as we have shown to 
be the case in Arius and in Platystoma, and other normal Pimelodime. 
On the whole the Weberian ossicles in Arius 2 Adada are very similar to those of 
Platystoma. The anterior process of the tripus (fig. 49, tr.a.) is somewhat shorter 
and of greater width than in most other Siluroids. The crescentic process {tr.c.) is 
rather thicker than usual, apparently in accordance with the greater thickness of the 
stratum of fibres attached to it, but on the ventral surface of its root the thickening 
abruptly ceases, leaving at that point a slight ridge, which, like the ventral ridge in 
Macrones, receives the insertion of a slip of fibres derived from the median portion of 
the anterior wall of the bladder. The crescentic process has no heel-like projection 
as in Macrones, i\ov is its posterior margin deeply grooved as in Platystoma. Claustra 
are present in the form of elongated spicular ossicles, slightly curved at their inferior 
extremities, and situated one on each side of the foramen magnum in contact with 
the posterior margins of the exoccipitals. 
The more important characteristics of the Arioid type as exemplified by Arius 
pidada may be summarized as follows : — 
[a.) The exceptionally firm anchylosis of the skull with certain of the anterior 
vertebrae, through the fusion of the greatly developed accessory articular processes of 
the basioccipital and complex centrum to form a stout subvertebral process, and the 
sutural union of the transverse processes of the fourth vertebra with the su])ra- 
occipital by the extension of a pair of strong osseous laminae between the two 
structures. 
(h.) The more extensive development of the superficial ossifications and their 
ventral and lateral extension to form complete bony canals for the dorsal aorta and 
the posterior cardinal veins. 
(c.) The absence of post-temporal plates and the exclusion of the post-temporal 
bones from contact with the air-bladder, the modified transverse processes of the 
fourth vertebra, and the subvertebral process alone forming an effective bony 
support for its anterior wall. 
(d.) The absence of compressor and tensor tripodis muscles, and of any tendency to 
