298 
adcauline sinus, the latter of which is in most cases divided int 
two lateral halves by means of a median projection. In each sinu 
is fixed an opercular membrane, the abcauline of which is in mof j 
species provided with a free distal valvular portion. 
The gonothecae present a very different habitus, being eithc 
smooth, ringed or provided with two or more spines. 
In most biserial species the aperture is distinctly turned towarc 
the frontal surface. 
In the large plurality of the species the adcauline sinus 
divided into two lateral halves by means of a more or less deve 
oped median projection. In all such cases the adcauline membrai 
is at the same time more or less distinctly angularly bent fro 
side to side, the ridge of the membrane rising from the medie 
projection. In all such cases the abcauline membrane is provid« 
with a free, triangular, valvular portion, fitting into the ang 
formed by the adcauline membrane, and the length of this porti( 
depends on the development of the median projection and the si 
of the angle in such a manner, that a more developed medi< 
projection gives a smaller angle and a longer valvular portio 
In such species as f. inst. S . argentea L, S. mirabilis (Ver 
and S. Birulae Schydl.*) the median projection and the angul 
bending of the adcauline membrane are only feebly developed whi 
they are well-developed in S. pumila and in all such species, 
which the hydrothecae of each pair are contiguous on the fronl 
side of the colony. Such species are f. inst. S. Versluysi Nu 
e. t. c. A still larger development is attained in S, tubuliforn 
(Markt. 2 ) in which species the median projection has the sai 
length as the lateral teeth, and the adcauline membrane is at le£ - 
of the same size as the abcauline. When a closed hydrotheca 
such a species is regarded from the side, the ridge of the adcauli 1 
membrane forms an obtuse angle with the adcauline wall, a l 
when regarded from the frontal surface its opercular apparat; 
might seem to be composed of three valves, two adcauline and i 
1 ) 55. 
2 ) 36. 
