9 
In many it would seem that the opercula move upon a 
sort of hinge, as in Myriozoum truncatum, fig. 15 ; Lepralia 
arrogata, fig. 6, and a large number figured. In others it 
seems that a membrane attached at the proximal end retains 
it in its place. The opercula of some are curved into a 
saddle-shaped form, as Hetepora Couchii, fig. 22, Eschara 
foliacea in stadium Eschara and Hemeschara, fig. 24, 25, 
and these have a concave proximal edge. 
In some the operculum consists of two separable layers, 
and this apparently causes the peculiar light portion of a 
part in Lepralia cucullata. In Tub ucell aria cereoides a 
light oval patch is seen in the centre, and in section fig. 36 
this is shown to be caused by the chitinous operculum being 
double elsewhere ; in fact it is like a meniscus with a con- 
cave stop in front. I have every reason to think there is no 
membrane enclosing this oval space, but cannot speak with 
certainty, as I have confined my attention to those points I 
deemed useful in determination, though a complete study 
of the structure of the opercula, generally, would no doubt 
repay the trouble. Tubucellaria hirsuta is not double in 
the same way, and has the long muscular bosses further 
from the edge. 
In a large number of the Bryozoa the old and young 
zooecia are known to vary so much as not to be recognisable 
as the same species if discovered separately, but the oper- 
cula of both arn the same ; also the operculated aperture is 
in many surrounded by an elevated peristome, in other 
words, is deep down in the throat, so that the shape cannot 
be seen on account of the projecting lips. In such a case 
the examination of the operculum will reveal the true form. 
The determination of the Celleporidse has seemed to be 
an almost hopeless task, as the form of the colony varies 
greatly, and the avicularia and other characters are difficult 
to use, but I believe the opercula may assist very much to 
bring this family out of its present confusion. In Cellepora 
