88 Mr. George J. Hinde’s Description of some 
preserved, moreover, the outer surface of these fossils is studded 
over with minute slightly projecting spines (PI. II., figs. 1, 8, 9), 
more readily perceived by a rasping sensation when the finger is 
rubbed over them than with a lens. 
The interior of the fossils is generally solid, for the canals and 
other microscopic interspaces have now been filled up by calcite, 
or by an infiltration of silica. The frequent occurrence of this 
latter substance has given rise to an impression that the fossils 
were originally of silica, but it is now detinitely known that when 
unaltered by fossilization the skeleton mesh-work is of carbonate 
of lime. 
A closer study of the structure of these fossils can only be made 
by means of sections sufficiently thin to be transparent under the 
microscope. These, however, as a rule, only show a confused 
mass of delicate fibres closely interwoven together, and until a 
clue is obtained to the character of the individual constituents of 
the fibres and the manner in which they are united together, it is 
very difficult to understand the nature of the organism. Asa 
consequence much division of opinion has arisen in the past 
among paleontologists respecting these fossils, and they have in 
turn been referred to various groups of the animal kingdom. 
By some fortunate circumstances, mentioned later on, some 
specimens were sent to me which proved conclusively that these 
fossils were sponges with a spicular skeleton of carbonate of lime. 
The spicules consist of four arms or rays (Pl. IL., fig. 5); one of 
these is elongate, tapering to a fine point, and furnished with 
lateral prickles or spurs; at the base of this long or apical ray 
there are three short, curved, divergent rays with blunt termina- 
tions. These individual spicules are connected together to form 
the skeleton of the sponge in such a manner that the elongate 
apical ray is always directed towards the outward growing surface 
of the sponge, and remains free or partially free (Pl. IL., figs. 1, 
8, 9); while the three short basal rays of the spicule are firmly 
welded by their truncate ends to adjacent spicules, and form a 
porous mesh- work which bounds the radial canals (PI.IL., figs. 1, 
2,4). The union of the basal rays is so close and intimate, that 
in microscopic sections of the fossils the individual spicules can 
be seldom distinguished (Pl. IL., figs. 6, 9), but they are shown 
on the surface of well-preserved specimens (PI. IL., figs. 2, 4). 
A very perfect specimen of one of these sponges in an early 
stage of growth was sent to me by Mr. H. Muller, of Eltham, 
Kent. It is hardly as large as an ordinary pin’s head, and is 
embedded in a fragment of flint. As shown in the figure (PL.IL., 
fig. 1), the outer surface is bristly, with the apical rays of the 
spicules all directed outwards, whilst the short basal rays are 
fused together. The further growth of the sponge is produced 
by the formation of successive layers of spicules, which overlap 
