52 Dr. H. A. Nicholson on the Minute Structure 
developed, and extend from the epitheca to the accessory wall 
surrounding the central tube, with which they become directly 
connected. The primary septa never, however, extend into 
the interior of the central tube ; and they alternate with well- 
developed secondary septa of more than half their own length. 
The calice is moderately deep, and exhibits at its bottom a 
small flat space formed by the upper end of the central 
tabulate tube. The free edges of the septa within the calice 
are denticulated ; and the cross section of the septa shows them 
to be intersected by conspicuous cross bars, these appearances 
being produced by a series of strong arched lamellar dissepi- 
ments, which are developed at corresponding points on the 
two sides of each septum, and are directed upwards and in- 
wards towards the centre. There is also a second series of 
more delicate dissepiments, which connect the septa with one 
another, are directed downwards and inwards, and give rise in 
longitudinal sections to a larger or smaller amount of exterior 
vesicular tissue. 
It will be seen from the above description, that in many 
respects there is a very close relationship between Crepzdo- 
phyllum and Heliophyllum. This is especially seen in the 
structure of the endothecal dissepiments, which are precisely 
the same in the two genera. In both we have a double series 
of dissepiments (figs. B & F), which intersect one another at 
high angles, those of the first series running upwards and in- 
wards, and those of the second series running downwards and 
inwards. In both, the dissepiments of the first series are so 
far peculiar that they do not actually connect contiguous septa, 
but have the form of strong curved or arched ridges, which are 
developed on the sides of the septa and at precisely corre- 
sponding points on the opposite sides of each individual sep- 
tum. Hence in both genera the dissepiments of this series give 
rise to three very characteristic and peculiar appearances : 
(1) the free edges of the septa in the calice are marked with 
blunt spines or teeth; (2) the sides of the septa, as seen in 
longitudinally fractured specimens, exhibit a series of pro- 
nounced striz or ridges, directed upwards and inwards in an 
arched manner, with their convexities upwards; and (3) the 
cross section of the septa, both primary and secondary, 
shows them to be intersected by conspicuous cross bars. In 
both Crepidophyllum and Heliophyllum, again, we find a 
second series of dissepiments, which are much more delicate 
in structure, and are directed approximately inwards and 
downwards, and which actually connect contiguous septa with 
one another. ‘These dissepiments are seen, in longitudinal 
sections, to form a series of comparatively large-sized vesicles, 
