Prof. G. Lindstrém—On the Operculate Corals. 138 
The genus Rhytidophyllum, with the single species R. pusillum, 
Lind., is a very minute form with a single semi-elliptical operculum. 
It is also from Gotland. 
In connexion with these distinctly marked operculate corals, 
Lindstrém describes two long-known species, Pholidophyllum tubula- 
tum, Schlotheim sp., and Syringophyllum organum, Linn., which 
possess on the outer surface of the corallum, peculiar exothecal 
structures, which, in a certain sense, are homologous with the 
opercula of the genera above mentioned. In the widely distributed 
P. tubulatum, the walls were covered with vertical rows of regularly 
disposed, minute, overlapping calcareous scales or plates, oval or 
pear-shaped in outline, minutely striated on the outer surface, and 
with a small process on the inner surface as if in connexion with 
a muscular attachment. The plates are in paired rows, each pair 
covering over the slight vertical furrow between two ruge or coste. 
Judging from thin sections, one side of each scale seems to have 
been slightly imbedded in the wall of the coral, but the attachment 
must have been slight, for examples in which these scales are 
preserved sn situ are very rare. 
In the other form, Syringophyllum organum, the exothecal scales 
are minute, convex, shield-shaped bodies, not more than -9 mm. in 
length and °8 mm. in breadth, which appear to have thickly covered 
the surface of the collar-like extensions which surround the corallites 
and form the horizontal floors uniting them together. They are now 
without regular arrangement, though it is probable that this is owing 
to subsequent disturbance. Lindstrém compares them with the 
small convex lamella of the peritheea in the existing genus Galazea. 
By some authors the concentric films or membranes which extend 
over the upper surfaces of some species of Favosites, such as F-. 
turbinatus, Bill., for example, have been regarded as of an operculate 
character, but in reality they have nothing in common with true 
opercula, and are merely of epithecal origin. 
In conclusion, Lindstrém points out that in nearly all the details 
of their organization the operculate corals approach so closely to 
other Cyathophylloid or Rugose corals, that they must be regarded 
as belonging to the same group with them, notwithstanding the 
peculiarity of their possessing opercula. The question remains as to 
the true zoological position of the whole group of the Rugosa, and 
Prof. Lindstrom shows that in the main features of their organization, 
the corals of this order resemble to a great extent the corals of 
Mesozoic, Tertiary, and recent periods. No great dependence can 
be placed on a comparison of the characters of the tabule or the 
vesicular laminzs of the interior of the coral, as similar structures 
are present in the hard skeletons of other vertebrate divisions. The 
general similarity of outer form is well known, and even the peculiar 
four-sided Goniophyllum pyramidale finds its parallel in the six-sided 
Flabellum Roissyanum, Ed. and H. As regards the presence of 
stolons, there are several species of Mesozoic and recent corals 
possessing these structures, and the same may be said of the vertical 
ruge. The septa in the Rugosa consist of a thin central lamella, 
