258 
PROFESSORS T. W. BRIDGE AXD A. C. HAD DON 
the first four vertebrfe with one another and their partial coalescence with the fifth, 
and the telescoping of the three anterior vertebrae and the associated portions of the 
Weberian mechanism through the foramen magnum on to the upper surface of the 
basioccipital, so that all these structures lie wholly within the cranial cavity and 
dorsad to the osseous recesses for the sacculi which here, as in the normal Siluridae, 
are j'oofed over by horizontal plates developed from the exoccipitals. With such 
singular modifications must also be mentioned the almost complete anchylosis of the 
conjoined and dislocated anterior vertebrae with the occipital portion of the skull, and 
the enclosure of the two completely separated air-sacs into which the air-bladder is 
divided within recesses formed by the so-called dorsal and ventral laminae of the 
modified transverse processes above, behind, and below, and by the posterior face of 
the skull in front, while the lateral or distal apertures of the recesses are closed by 
the lateral cutaneous areas only, the post-temporals merely forming their anterior and 
ventral lips. The absence of a sinus endolymphaticus, although the ductus endolym- 
phaticus still remains, the partially fibrous condition of the roof of the cavum sinus 
imparis, and the fact that, as a consequence of the forward dislocation of the anterior 
vertebrm, the scaphia form the side walls of the cavum sinus imparis instead of those 
of the atrial cavities, are also noteworthy features. Finally, may be mentioned the 
absence of a ductus pneumaticus, the partial ossification of the tunica externa of the 
inner half of each air-sac, and the fusion of the osseous cups so formed with the side 
walls of the neural canal in this region, and the presence of intercalaria and rudi- 
mentary claustra, in addition to a concavo-convex scaphium without condylar or 
ascending processes. 
The condition of the air-bladder, Weberian mechanism and the associated skeletal 
structures in II ypophthahnus are such as to justify the conclusion that in this genus 
the structures in question, while in some respects more normal than in any of 
the Hypostomatin£E at present examined, in others reach their extreme of retro- 
gre.ssive modification. In the relations of the post-temporals to the skull, to the 
pectoral girdle, and to the modified transverse processes, as well as in the fact that 
they do not close the external apertures of the osseous recesses for the air-sacs, 
llypophthalnius closely resembles the less modified Siluridse abnormales. A further 
resemblance to the latter is also shown by the enclosure of the two sacculi within 
bony recesses formed by the basioccipital and by horizontal plates derived from the 
exoccipitals, the partial roofing in by bone of the cavum sinus imparis, and the 
presence of intercalaria and claustra. The complete separation of the two air-sacs, 
the atrophy of the ductus pneumaticus, and the pos,pible suppression of the sinus 
endolymphaticus, may also be paralleled by similar retrogressive modifications in 
such otherwise less abnormal genera as Bagarius, Glyptosternum, &c. The mode 
of formation of the osseous capsules for the air-sacs is almost identical with the 
means employed for the same object in the Loricaroid genera. On the contrary, in 
