130 
DIASTOPOKA. 
of this species is partly, unilaminar and partly bilaminar ; in the 
former case the zoarial lamina occurs on one face, and the zoaiium 
is a Diastopora ; where it is bilaminar, the zoarial lamina occurs 
as a median layer, and the zoarium is a Mesnuteripora. The most 
instructive feature in the specimen (Fig. 7) is a young colony, 
which is in an almost Disco^par.'ia stage ; here the zoarium can be 
followed from the simple thin unilaminar condition, into the thick 
bilaminar Me>ienteripora condition with crowded zocecia and well- 
developed median zoarial lamina. 
Fig. 6. 
Fig. 6. — Part of a zoarium of Diastopora repens (Ilasw.), showing the 
unilaminar cup-shaped central portion, and part of a bilaminar frond with 
zoarial lamina ; x 8 dia. Recent : Australia. 
Fig. 7. — Part of vertical transverse section across a young frond of Diastopora 
repens (Hasw.), showing the zoarial lamiua ; x 16 dia. Recent ; Australia. 
As the ‘ germinal layer ’ is therefore apparently not of generic 
value, I see no reason for the retention of Mesenteripora as a genus 
distinct from Diastopora. Part of d’Orbigny’s Bidiastopora is also 
included in this genus. 
Thtaxia compressa, Hag., is a thick Diastopora, which gi’ew in 
shallow water, and has a thickly calcified zoarium ; the considerable 
development of the zoarial lamina is a natural correlation. In 
Diastopora marginata, on the contrary, the frond is very thin, the 
zocecia are long, and the apertures widely scattered ; accordingly 
the zoarial lamina is thin and inconspicuous, and the rudimentary, 
young zocecia — the ‘ geimies ’ of d’Orbigny — are few and scattered. 
