DR. G. J. HINDE ON RECEPTACULITIDA. 805 
of different lithological constitution, and also of the present nature 
of other fossils which are associated with them, and whose normal 
mineral structure is known. 
The different states in which the fossils now occur may be 
enumerated as follows :— 
(1) That in which the skeleton has been entirely removed, leaving 
only impressions and hollow casts to indicate its former presence. 
(2) That in which the skeleton is now in the condition of crys- 
talline calcite. 
(3) That in which the skeleton consists of iron peroxide and iron 
pyrites. 
(4) That in which the skeleton is of silica. 
Though, as a rule, individual specimens are exclusively in one or 
another of these mineral conditions, it not unfrequently happens 
that the same specimen may be composed of two or more different 
minerals, thus, for example, part may be of crystalline calcite and 
part of iron peroxide, and these two substances may even be inter- 
mingled in different proportions; in another specimen the skeleton 
may be partly of calcite and partly of silica; whilst, again, the 
structure of another may partly consist of iron peroxide and partly 
of empty moulds. 
(1) Considering the condition in which the solid parts of the 
skeleton have disappeared, leaving empty moulds, we find that this 
principally occurs in specimens preserved in calcareous shales and 
mudstones and in dolomites. Instances are met with in the De- 
vonian mudstones of Belgium; and the typical forms of Recep- 
taculites, described by Defrance, were partly in this condition, since 
he mentions ‘ trous ronds’ which appear to have been partly empty 
and partly filled with iron peroxide and calcite. In the calcareous 
shales and mudstones of the Wenlock and Ludlow strata of the 
West of England, the specimens of Ischadites Kenigu occur, with 
rare exceptions, as compressed casts, which rarely show more than 
the impressions of the summit-plates of the spicules and of the 
horizontal rays beneath. The other calcareous fossils in these beds 
with Jschadites, such as corals, brachiopods, lamellibranchs, trilo- 
bites, &c., have well-preserved calcareous skeletons. In the Galena 
beds or dolomites of Trenton age, in the States of [llinois, Iowa, 
and Wisconsin, the large flattened examples of Receptaculites occi- 
dentalis, Salt., generally known as the ‘“ sunflower coral” or “ lead 
fossil,” are entirely in the condition of casts. In these beds, also, 
even the shells of the brachiopods (with the exception of Lingula) 
and gastropods have disappeared, leaving only casts. It is a signi- 
ficant fact, however, particularly when we recollect the associa- 
tion of flint nodules and casts of siliceous sponges in the Upper 
Chalk of this country, that in the Galena beds of Illinois, where 
the casts of Receptaculites are so abundant, certain portions of 
the limestone are marked by an abundance of flints arranged in 
parallel layers*. 
* Geol. of Illinois, vol. i. p. 171. 
@.0.G.S. No. 160: 3H 
