25G 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
extensively developed in Clarias and AiUa. A special development of this process 
(“ventral process”) and the union of its posterior margin with the corresponding 
edge of the modified transverse process, while the anterior margins of both remain 
distinct from each other but blend with the posterior face of the skull, would 
eventually lead to the enclosure of the air-bladder within bony capsules exactly 
similar to those of the Loricaroid SiluridmP'' This explanation renders necessary the 
assumption that the modified transverse processes of the fourth vertebra have each a 
double origin from the vertebra in these Siluroids, and, moreover, explains the forma- 
tion of their osseous capsules as being due to the further extension of a modification 
already initiated in other and less aberrant S. abnormales. (3.) The imperfect closure 
of the saccular recesses and the cavum sinus irnparis by bone. The deficiencies of 
the Loricaroid forms in this respect must be regarded as due to the atrophy of the 
horizontal plates of the exoccipitals by whicli these cavities are usually roofed. A 
transitional stage in this process is apparently represented in Ilijpophthahnus. 
(4.) The suppression of the intercalaria and claustra. Although a constant character 
in those Loricaroid forms that liave yet been examined, the absence of one, or even 
both, of these elements is not unknown in the less abnormal members of this section 
of the family. 
The Hypostomatous genus Acanthicus, while in some respects resembling Loricaria 
and its allies, in others exhibits certain characteristic features of its own. In common 
with the preceding forms, the air-bladder is longitudinally constricted into two laterally 
situated oval sacs with structurally continuous walls, and connected by an intermediate 
tubular portion which, as in several of the Loricaroid genera, is supported by a 
“processus bijugus.” The presence of a ductus pneumaticus is uncertain. Eacli 
air sac is completely enclosed within an osseous capsule of corresponding shape, 
and, for a part of its extent, is situated dorsad to the first pair of ribs. What share 
the modified transverse processes take in the enclosure of the air-sacs is not clear 
from Reissxer’s account, but the shape and smooth rounded contour of the capsules 
certainly suggest that they are mainly, if not exclusively, formed by the ossification 
of the outer stratum of the tunica externa of the air-bladder itself, and that, unlike 
LoricaHa, Callichthys, &c., and Ilypophthalmus the transverse processes ca,n have 
but little share in their formation — a conclusion which receives some additional 
support from the fact that the bladder is everywhere in close contact with its bony 
investment, and connected therewith by fibres passing from one to the other. 
IIeissner seems to compare the “ processus auricularis ” with the distal extremity 
of a modified transverse process, which in Acanthicus, as in the Cyprinoid Cohitis 
fossilis, is perforated by certain apertures through which the external skin is pro- 
longed inwards and attached to the outer walls of the air-bladder ; but it may also be 
the case that the “ processus auricularis ” is really the post-temporal, which here, as 
in the Loricaroid types, closes what, in the dry skeleton, would otherwise be an 
* See also Sorensen’s account of Tlccostomns (37). 
