180 William Sörensen 
is necessary to operate the exact reverse of this twist i. e. that which 
is effected by the muscle inserted on sp, which muscle he has not 
seen either. But I must do him the justice to add that it is not to 
be seen in its full extent without part of »Humerus« being removed. 
In my Fig. 26, however, it is shown in its whole length. 
c) The process 0, which according to my view serves as being 
capable of fixing the ray by means of its scouring faces, is accord- 
ing to him part of the diarthrosis '. 
d) The process %, which according to my view carries part of 
the diarthrosis, is according to Dr. TuıLo (pag. 340) a »Hemmfort- 
satz«, which ». . . stemmt sich alsdann gegen die horizontale Kno- 
chenwand und verhindert durch Einklemmungen jede weitere Be- 
wegung«. (For the rest, with regard to the activity of the whole 
mechanism, I must refer to his own statement, pag. 340—342.) 
Which of these interpretations is correct — Dr. TuıLo’s or mine 
— of the two last points, which are of vital importance to the un- 
derstanding of the whole mechanism, this question it will scarcely 
be difficult for the reader to settle, when he bears in mind what 
follows: 
1. When the parts of the »Humerus« enclosing the process 0 
are removed (without touching any of those parts which in my opi- 
nion form the diarthrosis), the ray can no longer be fixed, but may 
be moved backwards and forwards with the same facility as the 
other rays of the fin or as the 1° ray of the pectoral fin of a Salmo, 
in which there is no such process. 
2. When the process ? is sawed off (without nothing else being 
touched), the ray may be fixed as well as before this operation took 
place. 
Though these two reasons must, I suppose, be pronounced suf- 
ficient, I will still add: . 
3. Examine the first ray of the pectoral fin of a fish, in which 
it cannot be fixed, f. i. a Salmo, a genus belonging to the same 
order as the Siluroide, and you will see (cf. Dr. TuıLo’s Fig. 43) 
lui donne encore une assez grande intensité.« — It ought to be remarked that 
the moving of the ray in Doras and Synodontis offers much greater difficulty 
in a recently killed specimen than in one which has for years been preserved 
in spirits of wine. (In the other genera the ray is much more easily moved.) 
1 pag. 340: »Der Gelenkkopf hat die Form eines Kegels«. And in the 
»Erklärung der Abbildungen«, pag. 355, the process d is designated as »Kegel«. 
