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the case in every undamaged hydrotheca. The one cause of Paar- 
man's mistake is that he has regarded these opercula only from 
the side and not from the abcauline surface. When such a closed 
hydrotheca is regarded from the side we get the impression of an 
A-tent, as we see a straight line limiting two membranes which 
might be regarded as two flaps, but this line is only the one side 
of the free abcauline triangular valve, which on closing fits into 
the corresponding sinus formed by the angularly bent adcauline 
membrane. The other cause is that he has studied these opercula 
by the aid of microtomic sections, as there can be no doubt that 
the cutting in many cases must have in different degree loosened 
the connection between the fine membranes and the hydrothecal 
margin. Therefore he has found that the degree of attachment 
between the membranes and the hydrothecal margin is very different 
even in the same hydrotheca, but as a rule both membranes have 
been torn away from the hydrothecal teeth (,,as a rule both flaps 
are functional"). 
Though I must, therefore, maintain the correctness of my 
earlier investigation I am willing to adwit, what I formerly denied, 
that the adcauline membrane may be regarded as a part of the 
Opercular apparatus, and that the operculum of Sertularia therefore 
may be called a two-lipped operculum. For this renewed investi- 
gation I have examined fresh material taken in the Trondbjems- 
fiord by Mr. 0. Nordgaard, and I have used the same mode of 
preparation as earlier, namely with a fine needle to cut off under 
the microscope the distal end of the hydrotheca and thereafter to 
examine it in different positions.  Colouring matter may help to 
make the membranes more distinct. 
After having expressed his agreement with Mr. Paarman's 
results Professor Nutting continues: "But there is still another 
and even greater objection to relying exclusively upon the characters 
of the margin and operculum in classifying the Sertudariidae, and 
that is that these characters are inconstant not only in some of 
its genera, but also in some individual species.” To prove the 
