ON THE DEVELOPMENT OF LICHENOPORA VERRUCARIA. 89 
completely above that septum to one beyond it. In this way 
is formed what may be called a compound alveolus; i.e. an 
alveolus which at its formation is partially subdivided by one 
or more vertical septa which do not reach the roof of the ovi- 
cell. The closure of the central part of the ovicell commonly 
and probably usually takes place by the formation of one or 
more compound alveoli, whereas the extension of the ovicell 
in a mature colony appears to take place for the most part by 
the formation of simple alveoli. 
The extent of the central area of the colony, devoid of 
zocecia, depends in the first instance on the extent to which 
the older zocecia diverge from one another; but it may be in- 
creased by the occlusion of zoccia. This process, which can 
be best made out in transparent preparations, has nothing to 
do with the occlusion of the fertile zoecium. It is highly 
probable that the occlusion follows the loss of the polypide.1 
The process does not appear to take place to any great extent 
in young and healthy colonies of L. verrucaria, but in old 
colonies a considerable number of the more centrally placed 
zocecia may be occluded. A calcareous lid may in these cases 
be added in the position of the original orifice (Pl. 7, fig. 9), 
and it may or may not be formed in an oblique position corre- 
sponding with the original condition of the orifice. This is 
very different from the state of an occluded fertile zocecium, in 
which the closure takes place at the level of the roof of the 
ovicell. 
All the older colonies of Lichenopora differ from those 
which have so far been described by the occurrence of a very 
striking process of secondary thickening of the surface. Since 
old colonies are further remarkable for producing new broods 
of embryos (cf. p. 132), which are developed in a different way 
from the first brood, the idea suggests itself that the second- 
arily thickened colonies are the result of a second year’s 
growth. I have no actual evidence in favour of this sugges- 
tion, but the fact that secondary broods of larve are produced 
1 Cf. my former paper on Crisia (5, p. 142). 
