2 
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 memhrane, 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, Memhraniporidae, Steganoporellidae^ Tha~ 
lamoporellidae. 
Åcanthostega. A 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 Crihrilinidae. 
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 Microporidae, 
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 Har mer 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, Reteporidae, 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 
M The three first groups are not sharply separated and nearer related 
to each other than to the fourth group. 
