48 Dr. H. A. Nicholson on the Minute Structure 
appear as successive rows of discontinuous ridges. Nor do 
they ever extend so far from any one septum as actually to 
reach the septa immediately contiguous to it. On the contrary, 
they occur in reality as so many strong ridges which are 
developed on the sides of each septum, and always in precisely 
corresponding positions on the two sides of any given septum. 
Hence it is that they appear in the calice as so many spines 
on the free edges of the septa, and in cross sections as so 
many cross bars intersecting the septa. Hence, also, in 
silicified specimens, in which the interior is exposed, they 
appear as curved striz or ridges on the otherwise plain sides 
of the septa; and this appearance is not due to any disappear- 
ance or destruction of the dissepiments subsequent to the 
death of the polype, but is really due to the inherent form of 
these structures. 
The dissepiments of the second order are exceedingly deli- 
cate, and are much less marked than those of the preceding 
series, which they intersect approximately at right angles. 
They are directed inwards and downwards, from the wall to 
the centre of the visceral chamber, and they are continuous 
between contiguous septa. ‘They form a series of lenticular 
vesicles, which are seen in longitudinal sections (fig. B) to be 
arranged in oblique rows, directed inwards and downwards, 
with their convexities upwards. The extent to which they 
are developed, however, varies greatly in different cases ; and — 
though they are always preeminently developed in the outer 
portions of the corallum, they are never present in such num- 
bers as to give rise to the conspicuous exterior zone of vesi- 
cular tissue which forms such a marked feature in corals such 
as the typical Cyathophylla. 
The genus Heliophyllum owes its name to the eminent 
American paleontologist, Prof. James Hall; but its first pub- 
lication was in Dana’s great work on the corals (op. jam cit.). 
It was originally regarded as nothing more than a subgenus 
of Cyathophyllum; and no higher rank is assigned to it by 
Dana than this. In reality, however, it cannot be placed 
even in the immediate vicinity of Cyathophyllum proper, with 
which it has hardly any characters in common. ‘Though this 
constitutes the first published description of the genus, it had 
been figured previously to this date, as the Strombodes helian- 
thoides of Phillips (‘ Paleozoic Fossils,’ pl. v. fig. 13, 1841) 
appears to be undoubtedly a species of Heliophyllum. 
The first description giving any thing like a really accurate 
conception of the structure of the corallum in the genus Helio- 
phyllum is that published by Milne-Edwards and Haime (Pol. 
Foss. des Terr. Pal. p. 408). They define the genus as 
