10 
the outside cells are decumbent, while the central ones are 
erect, so that often no similarity is apparent, but so far as 
my examination goes the opercula of both stages are the 
same. 
The size seems to vary but little, and in a slide of 6 or 7 
in many cases all would exactly correspond with the draw- 
ing of the one figured, and in few cases was the variation 
over a tenth of the size. I was also much surprised to find 
how closely the measurements of these Naples specimens 
corresponded with the sizes of the aperture as given by 
Smitt in his description of the Northern forms. 
Although only groups allied to Lepralia have now been 
brought under examination, it is by no means in these 
families only that the shape of the oral aperture is of 
importance, as in nearly all genera it is specifically charac- 
teristic as may be seen in Membranipora, Diachoris, Bugula, 
Flustra, &c., and a little more attention to this would have 
made the British Museum Catalogue of much more value, 
and prevented uniting under one name as Lepralia spinifera 
species with different shaped apertures, which it has since 
been necessary to separate. There are also other points 
which may be of use, as for instance I find in one Eschara 
from Australia that the operculum has 7 teeth in front, the 
central one the largest, and in Cellepora sardonica, fig. 27, 
it is strange to find lines on the edge corresponding with 
the minute teeth in the aperture. 
I should propose to divide the opercula into (a) those 
with a straight proximal edge 27, 28, 29, 30, 31, 34 ; (6) 
those with a straight edge and a sinal projection 1, 2, 3, 4 ; 
(c) those with a subtriangular proximal end 5, 6, 7, 8, 15 ; 
(cl) those with a rounded proximal end 9, 10, 11, 12, 16; (e) 
saddle shaped, mostly with a concave end 22, 23, 24, 25, 26; 
(/) suboblong 17, 18, 19, 20. 
That the opercula may be of the greatest use in specific 
determination there is now no doubt, and thus intimate 
