13 
zooecia of Diazeuxia hyalina, in the corresponding zooecia of Di- 
dymia simplex I have found a polypide of much smaller dimen- 
sions than those found in the other zooecia. 
2) The hyperstomial ooecia. With the exception of the o0ecia 
in Thalamoporella which must be referred to another type, we 
unite under this name all the external, two-layered o0ecia, seated 
over the zooecia or between two contiguous zooecia in the same 
longitudinal row. In most cases it is obvious that the inner layer 
(the endoooecium) can be regarded as a continuation of the distal 
wall while the outer layer (the ectoooecium) is formed from the 
front wall of the distal' zooecium. This however cannot be the 
case with the oocecia of Cellepora and Schismopora whose zo0ecia, 
produced by superficial, irregular- gemination, are erect and not con- 
nected wich distal zooecia. Here consequently both layers take 
their rise from the terminal part of the zooecium. An opercular 
apparatus is hitherto found in Bicellariidæ, Membraniporidae and 
in Microporella (Calvet!"). Tt is a membranous fold taking its 
rise from the anterior edge of the distal wall and in Membranipora 
and Microporella continuous with the posterior wall of the tenta- 
cular sheat. In Bugula, where an zooecial operculum is not deve- 
loped, it is continuous with the posterior lip of the zooecial open- 
ing. An atrium is sometimes present. The o0ecia belonging to 
this type we can divide into two groups, namely into oocecia with, 
and o0ecia without, a cryptocyst. 
Hyperstomial ooecia without a cryptocyst. To this group be- 
long the ooecia in the families Bicellariidae, Cellulariidae et On- 
choporidae. In the first family the oocecia can be characterized as 
free, their ectoooecium having only a very little basal part common 
with the front wall of the distal zooecium and otherwise projecting 
freely. While the endoooecium always seems to be calcified it is 
not always the case with the ectoooecium. In Bugula Murrayana 
f. i. the ectoooecium is for the most part membranous and only 
1) Op. cit. pag. 57, 169, 262. 
