21 
Bicellariidae and the Ctenostomata, support the view long ago 
proposed by Smilt, that the Cheilostoinata must be derived from 
the Ctenostomata. 
Cellulariidae. The distal wall consists of a posterior horizontal 
and an anterior ascending part, of which the former is provided 
with a group of rosette-plates with a single pore. Every lateral 
wall has two rosette-plates with several pores (to speak strictly 
a rosette-plate in the superior half and a hole in the interior half). 
Fixed, hypostegial ooecia. To this family belongs .^Flustra^' ar- 
mata. While all hitherto known avicularia are external, I have in 
Flahellina {Flabellaris Waters) roborata discovered an internal 
aviciilarium. In the posterior half of the front wall of this species 
there is generally found a single avicularium seated now in the 
right, now in the left side, very seldom two. In the zooecia pro¬ 
vided only with a single avicularium, I have always found an in¬ 
ternal avicularium seated exactly inside the empty place on the out- 
side of the front wall. 
Thalamoporellidae. From the nearly related Steganoporellidae 
(with which they have hitherto been put together and with which 
they agree in a number of characters, particularly in the structure 
of the cryptocyst), the Thalamoporellidae are distinguished by the 
possession of epistomial ooecia, vicarious avicularia and free cal- 
careous spicules in the form of bows and compasses. These spi- 
cules, which have been found in all the species examined (10) are 
seated partly in the zooecial (as well as in the avicularian and 
ooecial) cavity, partly in the space between the cryptocyst and the- 
covering membrane. The distal wall consists of a posterior hori¬ 
zontal and an anterior ascending part, of which the former is pro¬ 
vided with a semilunar group of single-pored rosette-plates. Each 
lateral wall has two rosette-plates with several pores. A simple 
operculum generally with a more or less concave posterior margin. 
Cellariidae. The members of this family differ from all other 
Cheilostomata in that the hexagonal areas in which the surface of 
the colony is divided, neither in the length nor in the breath corre- 
