810 DR. G. J. HINDE ON RECEPTACULITIDA. 
are now partly of calcite and partly of silica, and spicules of this 
genus even occur detached in the condition of calcite in the same 
strata in Gotland from which the calcite specimens of Jschadites 
have been procured. Evidence in favour of the siliceous constitu- 
tion of the Receptaculitide is also furnished by the circumstance, 
already alluded to, that the interior canals inthe spicules of thesiliceous 
examples of Receptaculites from the Trenton limestone are distinctly 
shown, whilst these structures have been entirely obliterated in 
portions of the same specimens which have been replaced by calcite. 
That some of the molluscan shells in the same rock have suffered 
a change from calcite to silica can be explained on the supposition 
that this silica has been derived from the solution of the portions of 
Receptaculites which have been replaced by calcite. The silica of 
the layers of flints in the Galena limestone of Illinois may also have 
originated from a similar source, as the numerous specimens of 
Receptaculites in these rocks are invariably in the form of hollow 
casts. 
In conclusion it may be stated that on comparing the present 
condition of the skeleton of the Receptaculitide with that of undoubted 
siliceous sponges from both Paleozoic * and Mesozoic strata, we find 
that there is a great similarity in their mineral structure and that 
it is now very frequently in the form of crystalline calcite, iron 
peroxide, iron pyrites, silica in a secondary condition as well as 
empty cavities, and there are therefore good grounds for believing 
that if the Receptaculitide are allied to Sponges, the proof for which 
must be sought from other considerations, their spicular skeleton 
was originally composed of silica. 
III. CHARACTERS OF THE DIFFERENT GENERA. 
Though the genera which I have included in the family of the 
Receptaculitidee all possess similar essential structural features, yet 
in some forms these are of a simpler character and are better pre- 
served than in others, and as these simpler forms furnish a key 
towards understanding the structure and character of the more 
complex, it seems desirable to consider them first. I therefore 
propose to commence with the genus Ischadites or Tetragonis, 
leaving Receptaculites to the last, since this genus possesses in certain 
respects a more complicated and variable structure than any of the 
others. 
Genus Iscuapires, Murchison, 1839, Siluria, p. 697. 
Tetragonis, Kichwald 7, Receptaculites, pars auct. 
The type forms of this genus consist of a group of flattened bodies, 
with circular or ovate outlines, from 25 to 40 millim. in diameter, 
preserved on the surface of a slab of hard bluish calcareous shale. 
* Since this paper was in type, specimens of Jschadites Kenigit have been 
discovered in the Silurian of the Pentland Hills, which closely resemble in their 
condition of preservation the examples of Amphispongia from the same locality. 
+ Urwelt Russlands, Heft 2, 1842, p. 81. 
