86 Prof, P. M. Duncan on Upper Silurian 



of one is embraced by the concavity of a neighbouring 

 spicule. 



Radial sections were made of some of the specimens, and 

 were examined by reflected, transmitted, and polarized light. 

 The dark-spot method of illumination was particularly suc- 

 cessful. The powers employed were from 30 to 450 diameters; 

 and the investigation was assisted by the action of dilute 

 hydrochloric acid as a solvent. 



The original hard parts are white and nearly opaque, and 

 when cut across and polished are almost homogeneous, only 

 minute granulation being visible under high powers. They 

 consist of carbonate of lime, and are white by reflected light, 

 and either colourless or light brown or grey by transmitted 

 light, the central dark-spot illumination producing an exqui- 

 site surface-opalescence. Polarized light produces but slight 

 colour; but it indicates a vast number of minute refractive 

 points : crystals of calcite are not seen, however ; and there 

 are no cleavage-planes in the hard original parts. The fossils 

 are infiltrated with clear transparent or rather dusky calcite 

 with very few cleavage-planes, and in some places giving indi- 

 cations, under polarized light, of a more or less acicular or 

 fibrous structure, like aragonite. Rhombs of calc spar exist 

 here and there ; and the intensity of the colours elsewhere, 

 under the crossed Nieols, varies much. Near the periphery 

 of the fossils, what may be termed orbicular calcite covers 

 the skeleton and intrudes in undulating contours on the homo- 

 geneous calcite ; and this margin acts differently on light to 

 the mineral on either side of it. In a few places the margin 

 looks almost membranous, if such a term is admissible, whilst 

 within it and near the skeletal elements the structure is some- 

 times acicular and close, or it presents the usual appearance 

 of infiltrated calcite. It is the projection of this orbicular 

 layer, with or without drusy cavities, which gives the ap- 

 pearance of a semiclosure of the canals here and there. 



On the whole, this remarkable layer is in the position of 

 the mass of dermal spicules in some recent Lithistids ; but no 

 trace of any can be seen. 



Radial sections through the centre of the fossils show that 

 there is a small space in the centre which is occupied by an 

 irregular reticulation of not universally continuous skeletal 

 elements. The meshes are wide, and are either without 

 any definite shape, or are quadrangular or pentagonal in 

 outline. In one section there are separate spicules in the 

 centre. 



From the edges of this central space a great number of 

 radiating, more or less straight canals pass to the periphery, 



