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adcauline sinus, the latter:of which is in most cases divided: into 
two lateral halves by means of a median projection. In each sinus 
is fixed an opercular membrane, the abcauline of which is in most 
species provided with a free distal valvular portion. 
The gonothecae- present a very different habitus, being either 
smooth, ringed or provided with two;or more spines. 
In most biserial species the aperture is distinctly: turned towards 
the frontal surface. 
In the large plurality of the species the adcauline sinus is 
divided imto two lateral halves by means of amore or less devel- 
oped median projection. In all such cases the adcauline membrane 
is at the same time more or less distinctly angularly bent from 
side to side, the ridge of the membrane rising from the median 
projection. In all such cases the abcauline membrane is provided 
with a free, triangular, valvular portion, fitting into the angle 
formed by the adcauline membrane, and the length of this portion 
depends on the development of the median projection and the size 
of the angle in such a manner, that a more developed median 
projection gives a smaller angle and a longer valvular portion. 
In such species as f. inst. $. argentea L, S. mirabilis (Verr.) 
and S. Birulae Schydl.7) the median projection and the angular 
bending of the adcauline membrane are only feebly developed while 
they are well-developed in S$. pumila and in all such species, in 
which the hydrothecae of each pair are contiguous on the frontal 
side of the colony. Such species are f. inst. S$. Versluysi Nutt. 
e. t. c. A still larger development is attained in $. tubuliformis 
(Markt.”) in which species the median projection has the same 
length as the lateral teeth, and the adcauline membrane is at least 
of the same size as the abcauline. When a closed hydrotheca of 
such a species is regarded from the side, the ridge of the adcauline 
membrane forms an obtuse angle with the adcauline wall, and 
when regarded from the frontal surface its opereular apparatus 
might seem to be composed of three valves, two adcauline and an 
hgpon 
