of Heliophyllum and Crepidophyllum. 47 
are seen in longitudinal sections (fig. B) occupying a central 
area of variable width. As a rule the tabulate area is of com- 
paratively small extent, and the tabule are somewhat remote 
and irregular ; but sometimes these structures occupy a con- 
siderable space, and are arranged with considerable regularity 
and close together. 
The septa (fig. A) are always very well developed; and 
both primary and secondary septa are invariably present, so 
far as I have observed. All the primary septa extend to 
the immediate vicinity of the centre of the visceral chamber ; 
but a large number of them, sometimes all of them, stop short 
of the actual centre. They all, however, become more or less 
flexuous as they approach the centre ; and, as a general rule, 
a certain proportion of them continue inwards till they become 
connected in a loose and irregular network, though they in no 
case form a central cellular mass. Nor is there, under any 
circumstances, any true columella. The secondary septa, 
again, are very well developed, and usually extend to at least 
half, or even two thirds, of the length of the primary septa. 
Lastly, both the primary and secondary septa exhibit in cross 
sections a variable number of conspicuous cross bars (fig. A), 
which give to them an exceedingly characteristic appearance, 
though this cannot be regarded as peculiar to the genus, 
These cross bars are confined to the exterior portions of the 
septa, and are wanting centrally. They are formed by the 
transverse section of the ascending dissepiments ; and as they 
run directly across the septa, it is evident that the dissepiments 
are placed at corresponding points on the two sides of each 
septum. Inno case, however, do the cross bars formed in 
this way extend from one septum to those directly conti- 
guous to it, but they are always confined to their proper sep- 
tum ; and they do not correspond in position in neighbouring 
septa. 
"The most characteristic features in the structure of Helio- 
phyllum are due to the very remarkable form and arrange- 
ment of the dissepiments—an arrangement which has been 
(but erroneously) supposed to be peculiar to this genus. There 
are two groups or orders of dissepiments (fig. B), which inter- 
sect one another nearly at right angles, those of the one series 
having an ascending direction as regards the corallum, whilst 
those of the other are descending. The dissepiments of the 
ascending series form a group of strong curved ridges, directed 
in an arched manner upwards and inwards from the wall 
towards the centre, with the convexities of the arches up- 
wards. When seen in longitudinal sections, they are never con- 
tinuous from the wall to the free edges of the septa, but they 
