Thilo’s memoir on »Die Umbildungen an den Gliedmaßen der Fische«. 185 
of the second ray. As Dr. Tui~o has not yet seen what in my 
opinion is the real diarthrosis of the second ray, I must observe 
that this diarthrosis is not shown on my figure. According to Dr. 
TuıLo my muscle »? (however only »nebensächlich«) is »ganz be- 
sonders geeignet zu verhüten, dass der Hemmknochen [the second 
ray} von seiner Gelenkfläche [the scouring face 0’) entfernt wird<, 
while I am of opinion that the very function of this muscle is to 
carry the ray out of contact with this face, if it is not pressed 
in against it by the first ray. For as far as I can see, »the most 
essential part of the fixation depends on the friction between the 
second ray and [the scouring face 0’ on] the interspinous bone«. 
Acanthurus chirurgus! Bl. The dorsal and anal fins. 
(My book, pag. 55—58.) 
If I have rightly understood the statement of Dr. TuıLo, then 
what he calls the »Achse« is the diarthrosis (or rather the articular 
face on the interspinous bone for the first ray). What according to 
him are the articular faces of the first ray and the interspinous bone 
are in my opinion fixing (or scouring) faces. Does is not appear 
somewhat strange for the face of a »Gelenkkörper« to be furnished 
with rough transverse rifles in Ac. chirurgus or, as Dr. THILO ex- 
presses himself when speaking of Ac. hepatus (see his Figg. 29 
and 32), to show »eine radiäre Streifung« ? 
To judge from the description of the structures of this spe- 
cies given by Dr. TuıLo the second ray in this fish must also 
be subject to fixation, as it is also indicated as being »angefügt« to 
the »Gelenkkörper«. — According to my opinion, the first ray is 
fixed by the simultaneous activity of the muscles M and m! (THILO’s 
Fig. 29); the fixation however cannot, in my opinion, be complete, 
except when the rays are carried quite forwards. 
The hindmost pair of muscles to the first ray has been over- 
looked by Dr. Tuto. 
The dorsal fin of Triacanthus, Centriscus, and Gasterosteus 
I cannot treat briefly. I therefore pass them over here, the more 
so as I do not think I have attained a full knowledge of the 
mechanism of these organs. I will limit myself to the remark that 
here too the spines are furnished with scouring faces. 
'T had only examined this species, while Dr. THıLo has examined Ac. hepatus. 
