50 Dr. H. A. Nicholson on the Minute Structure 
restriction of the tabule to a comparatively limited central 
zone, and to the fact that a certain number of the septa pass 
inwards to the centre, where they become more or less twisted 
together. ven in these points, however, the agreement is far 
from complete. In Cyathophyllum the exterior zone of vesi- 
cular tissue is invariably present, is largely developed, and is 
composed of very numerous minute cells; in Heliophyllum 
this zone is never largely developed, is sometimes altogether 
wanting, and is always composed of comparatively large cells, 
so as never to constitute a really conspicuous feature. Again, 
in the typical Cyathophylla the primary septa extend 
inwards to the centre, where they are twisted together so as 
to form a sort of spurious columella; in Heliophyllum, on the 
other hand, it is never more than a comparatively limited 
number of the primary septa which are continued inwards to 
the centre of the visceral chamber, and these, instead of be- 
coming twisted together, unite with one another to form a loose 
and irregular network. When, however, we come to examine 
the differences between these two genera, they are found to 
materially outweigh the points of similarity. The species of 
Heliophyllum are, more particularly, fundamentally distin- 
guished from those of Cyathophyllum by the presence of the 
peculiar arched lamelle which are directed inwards and up- 
wards along the sides of the septa, appearing on the free edges 
of the septa within the calice as so many teeth or spines, and 
constituting the characteristic cross bars by which the septa 
are seen in transverse sections to be intersected at regular in- 
tervals. No structure in any way capable of confusion with 
this has ever been detected in any Cyathophyllum. 
Lindstrém has suggested that Paleocyclus, E. & H., will 
probably be found to be allied to Heliophyllum; but I am 
unable to confirm this suggestion. The free edges of the 
septa in Palwocyclus ave denticulated in a manner super- 
ficially similar to what is seen in Heliophyllum ; but vertical 
and transverse sections show that this denticulation is pro- 
duced in a different way. At the same time the form of the 
corallum in Paleocyclus is quite unlike that of Heliophyllum, 
and the discoid forms are wholly destitute of tabule. 
The nearest ally to Heliophyllum is undoubtedly the genus 
Crepidophyllum. In this genus we find the central tabulate 
area of Heliophyllum and the same scantily developed external 
vesicular area ; whilst the free edges of the septa are rendered 
denticulate, and their transverse section is cross-barred by the 
same series of strong lateral arched lamelle. In many re- 
spects, therefore, we find a complete resemblance between 
Heliophyllum and Crepidophyllum. At the same time the 
