6 
Wall. While a number of Camarostega {Smittia, Ramphostomella, 
Umbonella verrucosa etc.) are provided with a similar simple oper- 
cular valve as most Malacostega, the chief bulk of that division 
possesses as the two named specie s a compose operculum, 
but as the compensation-sac in these forms opens immediately be- 
hind this, its proximal margin is not continued in the covering 
membrane of the zooecium, but in the posterior wall of the com¬ 
pensation-sac. In such an operculum we can distinguish between 
the valvular part and the accessorial part, and the hinge-line of 
the operculum forms the limit between these two parts. When the 
valvular part opens under the expansion of the polypide the acces¬ 
sorial part at the same time draws back into the zooecium and gives 
the water access to the compensation-sac. In a few cases ()S'c/i 2 >o/?oreZZ« 
Cecilii, S. circinnata, Megapora ringejis, Foveolaria elliptica) the valvu¬ 
lar part and the accessorial part are separated by a low non chitinized 
transverse band. In the two ends of the hinge-line the operculum 
is strongly connected with the corresponding margin of the orifice, 
generally in such a manner that a tooth-formed process from the 
later is coalesced with a chitinous ridge seated within the margin 
of the operculum. A similar connection is also found in Stegano- 
porella and Thalamoporella. Also the secondary orifice can some- 
times be provided with tooth-formed marginal processes which have 
been (for instance in Exochella longirostris and E. tricuspis) con- 
founded with hinge-teeth. While the hinge-teeth are connected with 
the inner surface of the operculum a median impaired extraoper- 
cular tooth is not rarely met with {Mucronella Peachii, Porella 
struma, Srnittia etc.). This tooth, as well as the median avicularium 
in many Camarostega seated immediately behind the orifice, no 
doubt serves to protect the entrance to the compensation-sac. In 
opposition to the compose operculum the simple operculum corre- 
sponds only with the valvular part of the former and differs only 
from the opercular valve in most Malacostega by being chitinized 
or (rarely) calcified and therefore separable. In Malacostega a 
simple chitinous operculum is found in some members of the family 
