Thilo’s memoir on »Die Umbildungen an den Gliedmaßen der Fische«. 173 
Among the muscles!, a! and a? serve to raise the rays Z and IT; 
no particular muscle is found to carry backwards 7; but the muscles p? 
carry both rays backwards — provided that the rudimentary 
ray is not fixed. »If a! and p? operate at the same time, they 
are not simply antagonists but @!, acting on a point, which is placed 
below that, on which p? is acting by means of the ligament (A’) 
between the two rays, they cooperate in swinging backwards the 
rudimentary ray [without this ray being moved out of its place], so 
that its naked faces are brought into contact with the likewise naked 
faces of the roof-like keel of the interspinous bone. But by this 
operation every movement backwards is rendered impossible, the 
rudimentary ray being fixed by the naked faces being pressed 
against each other; and any pressure from without on the spine? 
will only further increase the fixation by pressing the naked faces 
still tighter against each other. On account of the circular move- 
ment justly mentioned [in my book] of the rudimentary ray the fixation 
will, as may easily be understood, be stronger, the more the spine® 
is raised, feebler, the more it is pointed backwards. In this last 
instance the power displayed by p? will rather carry the naked 
faces along each other than in against each other; when the spine 
is placed in a quite erect position, the whole power will be nearly 
entirely employed to press the [scouring] faces against each other. 
The naked faces having once been brought into full contact with 
each other, and the rudimentary ray and the spine fixed by this 
operation, they will remain fixed against any movement backward, 
even if a! ceases to operate. However, you may always (with your 
hand) carry the spine as far forwards and then backwards again, as 
is the difference between the slack and tight condition of the liga- 
ment (4) which unites the rays; accordingly in Pseudaroides some 
distance, in Doras and Synodontis but very little.« (My book, pag. 13.) 
In another memoir* where I have given a very short account 
1 I] have examined the muscles only in Pseudaroides, Synodontis, and 
Doras. 
2 Or a pull at the spine by means of the muscles p?. I had not men- 
tioned this fact in my book, as I considered it superfluous. 
3 It is well-known that this ray (and the strong ray of the pectoral fin) 
is only a spinelike jointed ray; for brevity’s sake however I call it the »spine«. 
4 WILLIAM SORENSEN, Are the extrinsic muscles of the Air-bladder in 
some Siluroide and the »elastie spring<-Apparatus of others subordinate to 
the voluntary production of sounds? What is, according to our present knowledge, 
the function of the Weberian ossicles? A contribution to the biology of Fishes. 
