Ar ar, 
SPONGES OF JURASSIC STRATA. 
INTRODUCTION. 
In the series of rocks between the Yoredale beds of the Carboniferous 
Limestone and the Lower Lias there is an absence of fossil sponges in 
the British area, and throughout this interval (represented by the Millstone 
Grit, the Coal-Measures, the Magnesian Limestones and Red Marls of the 
Permian period, and the sandstones, marls, and shales of the Rheetic deposits) 
we can only suppose either that the conditions under which the strata were 
formed were unsuitable to sponge-life, or, what is perhaps more probable, 
that subsequent fossilisation has removed all traces of their existence. How- 
ever this may be, it is certain that not until reaching the horizon of the 
Lower Lias do we find sponge remains in any abundance, and even here these 
organisms are only represented by detached spicules. In some beds of the 
Lower Liassic Limestones of Glamorganshire, near Brocastle and the neighbour- 
hood, sponge-remains form, as pointed out by the late Mr. Charles Moore,’ an 
important constituent of the rock. The beds are filled with spicules, in form 
resembling those of the still existing genus Pachastrella, Os. Schmidt, but their 
original silica has been removed and they now consist of carbonate of lime. 
Similarly some beds of limestone on the same geological horizon, near Shepton 
Mallet, Somersetshire, are crowded with detached spicules of siliceous sponges 
now replaced by calcite (Pl. XIII, fig. 4). At Harptree, in the same neighbour- 
hood on the Mendip plateau, there are beds of chert of some considerable 
thickness belonging to nearly the same horizon as the limestones, which may very 
probably have been derived from the organic silica of sponges, although, with one 
or two exceptions, spicules are not recognisable in them. 
1 «Quart. Journ. Geol. Soc.,’ vol. xxiii, 1867, p. 538. 
BB 
