176 William Sörensen 
that the more acute the angle is, formed by the scouring faces, the 
better adapted they are to serve in fixing the rudimentary ray. May 
I be allowed to say, in all modesty, that in all its plainness, this 
information appears to me more valuable than even the most exact 
measuring of the angle, when we do not know what is the impor- 
tance of the absolute aperture of the angle. 
In one place, it is true, I had veiled my opinion in some degree, 
when I said (pag. 15) in reference to the (first) paper of Dr. THILO, 
that »it does not betray much knowledge as to the mechanism of 
the fixatione. For I cannot deny that by this remark I meant to 
say that he had not understood it at all. 
Whether Dr. THiLo’s interpretation of the mechanism of these 
parts or mine be correct, may be judged, I suppose, from what 
follows. The effect of the muscles p? may be imitated by bending 
the spine backwards with your hand; in both cases a pull at the 
rudimentary ray is effected. In a Synodontis or other Siluroid, 
preserved in weak spirits of wine, the muscles of which have not 
become rigid, the spine may without any difficulty be carried back- 
wards and forwards with your hand! (or moved with the end of 
the finger, placed at the point of the spine). If, on the contrary, 
the point of the rudimentary ray is pressed downwards?, it is 
impossible to lay down the spine, because the scouring faces (which 
according to Dr. THıLo were to be articular faces) are brought into 
full contact with each other. (Of course this may also happen 
accidentally, when the spine merely is carried backwards and 
forwards with your hand.) If in such a specimen the rudimentary 
ray and consequently the spine be fixed, the fixation will cease 
when the point of the rudimentary ray is lifted upwards, by which 
operation the scouring faces are removed from each other*. (The 
1 That this must be an impossibility appears to me to be the meaning 
of Dr. THiLo’s words (pag. 312—313): »Zieht man mit einer Pincette an den 
Sehnen der Muskeln M und m [mine a! and a2], so richtet sich der Stachel 
auf und ist durch einen Druck gegen die Spitze des Stachels nicht niederzulegen. 
Wohl aber gelingt das Niederlegen, wenn man an der Sehne des Muskels m’’ 
[my p2] zieht oder mit der Pincette die obere Spitze des Strahles 1 erfasst und 
nach hinten zieht.<« 
2 That I may not run the risk to be misunderstood, I perhaps ought to 
observe that by this operation the point must not be turned forwards. 
3 In a specimen preserved in spirits of wine the fixation may happen to 
cease, when you cease to press downward the point of the rudimentary ray. 
