730 
EDWAED AV. SHANN. 
ray, for in that case it would have reversed its coui’se of 
evolution to a quite unimaginable extent. It is equally 
unlikely that it is a rudimentary fiu-ray, or part of a fin-ray, 
since it contains but one large scale (instead of a series set 
eud to end), and further, it lies dorsal to all parts of the 
fin skeleton. It is alienated from any fundamental resem- 
blance with the adipose dorsal fin, in the first place because 
it develops comparatively late in the life of the fish, it is an 
adventitious outgrowth, that is to say, not the result of the 
development of a pre-existing embryonic fold (as is the case 
with the adipose dorsal) ; in the second place its fatty matter 
is of a different composition, and it is devoid of horny rays 
(actinotrichia). It seems probable, then, that the adipose 
lobe of the pelvic fin of Sal mo is an organ sui generis. 
This does not detract, however, from its impox’tance as 
sugg'estiug the lines by which a fin may have been derived 
from a scaly outgrowth of the body-wall. 
The function of this remarkable structure presents a puzzle 
to the investigator. It is almost impossible to believe that 
an organ of such large dimensions and regular occurrence in 
the genus Salmo serves no useful purpose in the daily round 
of these fishes. It seems justifiable to dismiss summai’ily the 
idea that it is a storage organ; the relatively stable properties 
of its fatty matter, the development of a stiffening axis, and 
its invariability in salmon of varying physical condition all 
point in this direction.^ Again, as it is equally developed in 
both sexes, it is probably not analogous with the “ claspers ” 
of Blasmobranchs. Gunther has laid stress on the value 
of the paired fins of fishes as balancing org-ans.^ The pectorals 
and pelvics ai’e placed where they are required to support the 
greatest weight of the fish on which they occur ; thus the 
salmon, being thickly built in the postei’ior abdominal region, 
requires large ventrals. The adipose lobes may then act as 
additional balancing organs for the pelvic region ; further, 
situated as they are just in the outer angle of the pelvic fin 
* Except in one extreme case, see p. 709. 
^ ‘ The Study of Fishes,’ p. 44. 
