186 William Sörensen 
Triacanthus. The ventral fins. (My book, pag. 69—71.) 
Dr. TuıLo and I agree as to what is the diarthrosis and as to 
its form. Also as to the fixation being operated by the »Hemmfort- 
satz«. But Dr. Tuito has not seen, that on the interior sloping 
edge of the »Hemmfortsatz« there is one and on the »pelvis« two 
scouring faces, although they are not only mentioned in my text but 
also shown and very distinctly on my Figg. 32 & 33. And, accord- 
ingly, he has not seen either that the spine can be fixed only in 
two positions, viz. when the scouring face on the »Hemmfortsatz« 
is brought into contact with either one or the other of the scouring 
faces of the »pelvis«. The muscle! by which the spine is fixed 
when in these two positions, is capable of raising it when the 
scouring face on the »Hemmfortsatz« does not touch a scouring face 
of the »pelvis«. 
That the said scouring faces are found in the species examined 
by Dr. Taito I know as I had myself examined this one too. But 
it is quite natural that he must have failed to see them, for ac- 
cording to Dr. TuıLo the scouring faces are articular faces wherever 
they are found, and it would be unreasonable to suppose that the 
end of the »Hemmfortsatz« could articulate with the »pelvis« in 
only two positions. 
Monocentris japonicus. The ventral fins. 
I myself have not dissected this fish, but Dr. Rırum who has 
treated its ventral fins, has been so kind as to lend me a prepa- 
ration. I have not fully understood the mechanism; but however I 
have seen enough to 1) confirm the statement of Dr. TuıLo that the 
fixation may be made to cease by a torsion of the spine viz. when 
its posterior edge is twisted downwards while the spine is car- 
ried backwards, whereas it is fixed, so that this movement cannot 
be effected, when its posterior edge is twisted upwards, 2) further 
that it is the »Radscheibe« of Rrenm (the »Scheibe« of TmıLo) which 
is scoured against the »Lager« of Rieum while the »Haken« of 
RıEHM (the »Hemmscheide« of THILo) is scouring against the »Zahn« 
of RıeHm. — Is any part to be particularly designated as a »Hemm- 
fortsatz«, then in my opinion it must be the »Radscheibe«. 
' Its place is marked by the line M in TurLo’s Fig. 33. 
