806 DR. G. J. HINDE ON RECEPTACULITIDA. 
The aspect of the casts of Receptaculites and Ischadites differs so 
much from that of the same fossils when the structures themselves 
are present that one can readily understand the origin of the erroneous 
opinion held by several paleontologists that these fossils consisted of 
series of hollow cylindrical cells. In the examples from the Galena 
limestones of Illinois, the surface of the fossil is divided into shallow 
lozenge-shaped areas by the crossing of comparatively thick curved 
ridges of yellowish coarsely-granular dolomite. At the bottom of 
these areas are the hollow cylindrical tubes, the casts of the vertical 
spicularrays, which extend quite through to the hcrizontal plate above. 
The tube-walls, or rather the interspaces between the tubes, for 
there are no definite walls, consist of the same dolomitic material 
as the matrix. When the under surface is worn down or broken 
away only the cylindrical tubes are exposed. The only indications 
in these specimens of the horizontal rays of the spicules are four 
small horizontal projections, which extend from the cell-walls 
immediately beneath the cast of the summit-plate. These projecting 
processes fill the interspaces originally existing between the horizontal 
rays. The coarse-grained character of the dolomitic matrix of these 
fossils is ill-adapted for retaining their finer structural markings. 
The casts of Jschadites from the finer calcareous mud of the 
‘Wenlock and Ludlow strata vary considerably in appearance from 
those of the fossils just referred to. In the best-preseryed specimens, 
which, however, are more or less distorted and flattened by compres- 
sion, the surface is marked by distinct ridges which cross each other 
diagonally, leaving between them small lozenge-shaped or rhom- 
boidal depressions. The ridges are usually even, though sometimes 
they exhibit minute sinuosities. Occasionally the depressions are 
crossed by two other series of lines, less distinctly marked than 
the diagonal ridges, which divide each rhomboidal area into four 
triangles. At the intersection of these lines, in the centre of each 
area, are small circular apertures, not always shown however, which 
are the casts’ of the vertical spicular rays. The diagonal ridges 
represent the minute linear interspaces between the margins of the 
head-plates of the spicules; and the vertical and concentric lines, 
which are more clearly shown when the diagonal ridges are worn 
down, indicate the horizontal rays. 
(2) In which the skeleton consists of crystalline calcite. This is 
the common condition in which these fossils occur, and it is found 
in specimens from different formations and localities. Thus calcite 
is present in /schadites from Gotland and in some forms of the same 
genus from Dudley and Malvern ; it forms the skeleton of Sphero- 
spongia from Devonian strata in Devonshire, also of Acanthochonia 
from the Silurian rocks of Bohemia, and of Receptaculites from the 
Trenton limestone in Canada, from the Silurian strata of the Baltic 
and the Arctic regions, and from the Devonian of Silesia and Belgium. 
The matrix enclosing these crystalline calcite specimens is usually 
an organic limestone in which the remains of other kinds of fossils 
are abundant, and these, for the most part, retain the calcite in an 
amorphous condition; in other cases the matrix is a calcareous 
