PROF. P. M. DUNCAN ON SLEREPTELASMA RG@MERI. WG 
phyllum; but they insisted upon the generic value of the absence 
of epitheca, and relied upon the feeble development of the dissepi- 
ments and tabule. 
We owe the first notice of the occurrence of the genus in Great 
Britain to Messrs. Nicholson and Etheridge, who describe species 
and carefully consider the generic characters of Streptelasma in their 
valuable work on ‘The Silurian Fossils of the Girvan District’ 
(p. 67). They amend the generic diagnosis, and show that the 
differentiation must rely upon the union of the inner ends of the 
septa to form a false columella, and upon the feeble amount of 
endothecal tissue*. I agree with these authors in every important 
particular, and admire their careful and elaborate work very much. 
Any difference of opinion which may occur between us relates 
rather to terminology than to matters of fact. 
The structure of the theca or wall of the Rugosa, and its homo- 
logies with the theca or epitheca of the Aporosa and Perforata, have 
been fertile subjects for discussion. Messrs. Nicholson and Etheridge 
write, ‘‘Hpitheca well developed. Proper wall doubtfully present, 
but a thick false wall, formed by the fusion of the broad outer ends 
of the septa with intermediate calcareous deposit” (op. cit. p. 67). 
There is no doubt that the wall in many Rugosa and in Streptelasma 
resembles an epitheca in its external appearance, and it is not 
separable from a subjacent structure like a theca. The delicate 
transverse lines of ornamentation are exactly like those of many 
species of Aporosa, where there is a structure separable from the true 
wall or theca, and which is called epitheca. They resemble, however, 
those ornamental lines seen on species of Zaphrentis, where a thickish 
mural structure is covered here and there with what resembles an 
epitheca closely. 
In the Aporosa the wall is certainly formed by an extension of 
septal structures, and it appears after the septa in the embryonic 
state ; and the epitheca, a most variable structure, is developed after 
the wall, for it often covers coste and their spines. It often appears 
to be produced by a basal tissue which reaches up to near the calice. 
But in some Aporosa (in some species of Mlabellum, for instance) 
the epitheca is pellicular, and forms an intrinsic part of the wall. 
Again, in Phymastrea the epithecal processes, distinct enough in 
some parts, blend with the true thecal structures elsewhere. 
For purposes of classification the epitheca is comparatively of only 
specific or subgeneric value; and it would lead to confusion, unless 
there were better reasons for so doing than now present themselves, 
to employ the term epitheca for theca. 
Again, the ridges and furrows on the outside of Streptelasma are 
plainly the analogues of the coste and intercostal spaces of the true 
Corals, and should be thus termed. 
In the instances where the costal interspace or groove corresponds 
* In the drawing of Streptelasma europeum, by F. Romer, the tabular struc- 
tures are extraordinarily develo ed, and the septa curve right over the axis. 
Probably there is an error; for it is not easy to see how the succession could 
occur. 
