DR. G. J. HINDE ON RECEPTACULITID A. 807 
shale or mudstone. In no instance, so far as I am aware, has any 
member of this group been discovered in arenaceous strata. 
In examples of Jschadites from Djupvik in Gotland, as also in 
some forms of Spherospongia and Acanthochonia, the head-plates of 
the spicules are exceptionally well preserved, and exhibit a smooth 
shining horny lustre so as to give an impression that they are 
formed of amorphous calcite. This smooth appearance is, however, 
merely confined to the outer surface of the plates, for in transverse 
sections the material immediately beneath the surface is decidedly 
 erystalline. As a general rule the outer surface of the spicules, 
when laid bare by weathering, is slightly rough and uneven, and the 
internal aspect in polished sections or fractured surfaces is more or 
less coarsely crystalline. The interspace between the outer and 
inner surface is usually filled with a matrix of a similar character 
to that of the enveloping limestone rock, and in this the vertical 
spicular rays, as seen in sections, show well-defined outlines, but no 
special wall-substance is apparent between the matrix and the 
crystalline substance of the rays. In some instances the summit- 
plates and the horizontal rays are amalgamated into a continuous 
mass of crystalline calcite in which neither canals nor the separate 
horizontal rays can be distinguished. In these cases it would seem 
as if the original material had been first completely removed by 
solution and the moulds afterwards filled with the crystalline cal]- 
cite. In none of the Silesian specimens of Receptaculites which I 
have seen is there any indication of the finely fibrous crystalline 
structure which Giimbel regards as the original character of the 
skeleton, though in some of the weathered examples of Acantho- 
chonia from Bohemia, the crystalline calcite is fibrous and the 
erystals radiate from centres in a somewhat similar manner to that 
figured by Gimbel. The calcite in weathered-out specimens of 
Spherospongia from Devonshire is mostly of a granular crystalline 
character, and the surface of the spicular plates and rays is peculiarly 
rough and uneven. 
(8) In which the skeleton consists of iron peroxide and iron 
pyzites. The former material is of very common occurrence in 
specimens of Receptaculites from Belgium, Silesia, and Canada, and 
of Ischadites from Gotland. The matrix of these fossils is either 
limestone or calcareous shale. The instances in which the skeleton 
consists of iron pyritesarerare. Not unfrequently the iron peroxide 
is partly intermingled with calcite. It is sometimes of a dark 
brown, but more usually of a rusty tint. As a rule the head-plates 
are not clearly shown in specimens of this material, and when the 
outer surface is preserved, it appears as a thin continuous dark or 
rusty crust. When this is slightly weathered, the outlines of the 
* plates are indicated by delicate lines of the fine-grained matrix; 
further weathering exposes the horizontal rays of the spicules, form- 
ing, in the examples of /schadites, clearly marked dark lines running 
from the base to the summit crossed by other lines tracing concentric 
circles. When more closely examined, the lines are seen not to be 
in all cases continuous, as the spicular ray sometimes do not meet, 
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