808 DR. G. J. HINDE ON RECEPTACULITID. 
though very frequently they overlap each other. In specimens still 
further worn, by weathering or other causes, the horizontal rays 
disappear, and the surface is covered with circular apertures, infilled 
with the iron peroxide, disposed in straight lines running from the 
nucleus to the summit or the periphery of the fossil. The rusty infil- 
ling is frequently removed by the weather, and the holes themselves 
become considerably enlarged. Though the horizontal and vertical 
spicular rays are very clearly shown in these iron-peroxide spe- 
cimens, yet { have not detected in them any traces of the interior 
canals, nor, in fact, could these structures be expected to be preserved 
in such soft incoherent material. 
(4) In which the skeleton is of silica. The only examples in 
which, to my knowledge, this mineral constitutes the skeleton are 
specimens of Receptaculites from Trenton limestone at Pauquettes 
rapids, on the Ottawa River. The matrix enclosing these specimens 
is a compact blue limestone filled with the remains of corals and 
mollusca, some of which at least are replaced by silica in the form 
of Beekite. The silica in Receptaculites is evidently in a secondary 
condition, as it appears, after the matrix has been removed by dilute 
acid, in a minutely granular state on the exterior surface of the 
plates and rays, giving them a rough aspect under the lens, and in 
‘the form of delicate plates and radiating crystalline fibres in their 
interior. It happens even here that the specimens are not invariably 
of silica, but part of an individual may be of crystalline calcite and 
part of silica; and it is a notable circumstance, as tending to throw 
some light on the original material of these bodies, that whilst in 
the siliceous portion of the specimens the interior canals are clearly 
shown in the spicular rays, no traces of them appear in those parts 
which have been replaced by crystalline calcite. 
From the aboveitisevident that the question of the original mineral 
nature of Receptaculites and its allies is a complex one, and that it is 
doubtfulif a single specimen has yet been discovered in which the ori- 
ginal structure has beer preserved. It will I think be generally con- 
ceded that crystalline calcite, of which the skeleton is now most fre- 
quently composed, does not constitute its primary structure, which must 
either have been another form of carbonate of lime, such as aragonite, 
or silica. Gimbel * maintains that in certain specimens from Silesia 
the original structure of Receptaculites exists in the form of a finely 
fibrous crystalline material which may be taken to be aragonite, 
but he does not mention the characters by which it may be distin- 
guished from calcite. The proof brought “forward by Gimbel that 
this material forms the original structural element of the skeleton 
is that the constant oblique direction in which the crystalline fibres 
radiate from the central axis of the vertical spicular rays never 
appears in purely crystalline structures, and also that there are 
numerous parallel lines within the fibrous material which cannot be 
attributed either to aragonite or to fibrous calcite, but to organic 
structure ; this material, in thin microscopic sections, has a great 
resemblance to the ‘prismatic layer (Kalkstabchenschicht) in the 
* Beitr. p. 26 (sep. copy). 
