view of the calcified front wall of those Cheilostomata provided with 
a compensation-sac. Before entering into this question I propose 
to divide the Order Cheilostomata in. the following four groups: 
Malacostega. The front wall in a greater or lesser part of 
its surface only consists of a membrane, by two rows of parietal 
muscles connected with the calcareous side-walls; the operculum as 
a rule is a membranous valve; no compensation-sac. To this di- 
vision belong the families: Aeteidae, Flustridae, Farciminariidae, 
Bicellariidae, Cellulariidae, Membraniporidae, Steganoporellidae, Tha- 
lamoporellidae. 
Acanthostega. AÅ greater or lesser part of the front wall is 
membranous and protected by a roof, formed by the overarching 
and fusion of numerous circumareal spines; the parietal muscles 
as in Malacostega ; no compensation-sac. Family Oribrilinidae. 
Coilostega. The calcified front wall is more or less concave 
and surrounded by raised margins; a chitinous operculum; no com- 
pensation-sac. To this divison belong the families MMicroporidae, 
Chlidoniidae and Cellariidae. 
Camarostega. The calcified front wall is more or less convex 
and not surrounded by raised margins; the operculum is chitinous 
or membranous; a compensation-sac"). 
This group which is identical with the Lepraliod or Escha- 
rine forms of Harmer contains the chief. bulk of the Cheilostoma- 
tous Bryozoa and is that whose division in families and genera pre- 
sents the greatest difficulties. It complects the following families: 
Onchoporidae, Calymmophoridae, Adeonidae, BReteporidae, Cateni- 
cellidae, Porinidae, Mucronellidae, Smittiidae etc. 
Before entering into the question of the front wall I think it 
useful to propose some names for the different types of individuals 
which can be met with in the colonies of the Bryozoa. Besides the 
1) The three first groups are not sharply separated and nearer related 
to each other than to the fourth group. 
