100 
Transactions of the Royal Society of South Africa. 
128 mm. in length, it is readily seen by raising the membranous border of 
the operculum, which conceals it. It consists of a transparent fold of skin, 
which at its upper end is continuous with the opercular membrane. It 
attains its greatest breadth (To mm.) about 2 mm. from the upper angle 
of the opercular opening, and from this broader part gradually tapers away, 
the total length being 7 mm. The whole structure is situated over the 
clavicle, to which, however, it is not attached, and that it represents a 
reduced pectoral fin is indicated by the presence of six dermal rays, though 
no traces of other elements (radials) of the limb are to be seen. These 
dermal rays are widely set apart, and are so arranged as to form a support 
for the elongate fold. The rays taper off towards the border, near which 
they end. This border is slightly emarginate between the distal ends of the 
rays, the emargination being much more marked in movement in the 
living condition (PI. IV, fig. b) . The two halves of each ray are fused together 
throughout their length, except at the proximal end, where they appear as 
short stout forks, the function of which in normal fins is to secure the rays 
to the radials. As these radials are absent in this case, the meaning of the 
marked development of these forks here may be interpreted as a device for 
the more effective control of the valvular pectoral in its movements. 
The upper and lower ends of this fin may be particularly noticed (PI. IV, 
fig. a). At its upper end it is distinctly continuous with the margin 
of the opercular membrane, and but for the presence of the rays the whole 
structure might be interpreted as an extension of this margin on to the body. 
At its lower end it is continued as a rayless membrane, gradually becoming 
narrower. This rayless part is about half its total length, which is 
considerable, being about a third of the whole opercular opening. 
This structure differs very considerably from the normal fin of fishes. 
The mesodermal supporting elements of the fin have been lost, the dermal 
rays reduced in length, though comparatively stout; their proximal ends are 
well developed and spread out. Though the fin is shorter in length, its base 
is relatively large, so as to form a thin vertical membrane, which is supported 
by the altered position of the rays. Not only has the structure of the fin 
been very much altered, but the function is totally different. 
We have therefore here an illustration of the appearance of a new organ, 
which apparently does not occur in any other fish,* and is of comparatively 
recent origin. 
It can be accounted for, as any modification of the body can be accounted 
for, by a process of natural selection of favourable germinal variations. 
Such an explanation cannot, however, with our present knowledge of cell 
structura and function, either be proved or disproved, and we may meantime 
look round for some other possible suggestions. 
# Since this paper was in print, I have noticed a similar fold, but without rays, in 
some of the Chimaeridae, and that, in the living Callhorhynchus, it functions as an 
accessory valve in respiration. 
