RHAXELLA PERFORATA. 211 
plates or trabecule from *5 mm. to 4 mm. in thickness, disposed so as to leave 
irregular channels and open spaces between them. The outer surface of the wall 
is smooth or slightly ribbed, and it is perforated by ovate or narrow slit-like 
apertures from 1 mm. to 9 mm. in width, whilst the interspaces between the 
plates, as seen in a transverse section of the wall, vary in width from 1 mm. to 
about 4 mm. (Pl. XIII, fig. 7). The wall-plates are entirely composed of small 
globate spicules, closely aggregated together (Pl. XIII, fig. 7c). They are 
ellipsoidal or subspherical in form, with a small depression or hilum, which gives 
them a kidney-shaped appearance; in size they range between ‘11 mm. and 
‘15 mm. in diameter; some smaller forms are, however, not more than ‘08 mm. in 
thickness. As in the globates of existing species of Geodia and Placospongia, 
these fossil forms are composed of numerous minute spicular fibres or rods in 
close contact, which radiate from the centre of the spicule and terminate at the 
surface as small subcircular spots, (002 mm. in width, regularly arranged in 
quincunx (PI. XIII, figs. 7e, 7 f). Many of these globates are nearly as perfect 
as those of existing sponges, and their structures are readily seen when examined 
in glycerine under the microscope, but the surfaces in others are extensively 
corroded, and the central portion in these is often changed into banded layers of 
chalcedony. The spicules are now usually cemented together to form the wall- 
plates by a secondary deposit of silica, and in some instances in the central 
portion of the wall their distinctive forms have been obliterated, and they are 
merged in a mass of chalcedony. 
The skeleton in these sponges as now preserved to us appears wholly to 
consist of the globate spicules, and in this respect they markedly differ from all 
other sponges. In Placospongia, Gray, the nearest allied genus, there is, in 
addition to an inner axis and surface plates of globate spicules, an intermediate 
skeleton of pin-shaped spicules which seem to be wanting in this fossil form. 
Connected examples of this species are rare, but detached globate spicules of 
a similar character to those composing these sponges are extremely abundant, so 
as to form in some instances the principal constituents of beds of cherty rock of 
considerable thickness in the Lower Calcareous Grit of Scarborough and the 
neighbourhood. They were first noticed by Dr. H. C. Sorby (‘ Quart. Journ. 
Geol. Soc.,’ vol. vii (1851), pp. 1—6); (‘Proc. Geol. and Polytechnic Soc. 
Yorkshire,’ vol. ii (1851), pp. 197—206, Pl. IV), and subsequently by Mr. J. F. 
Blake (‘Monthly Microscopical Journal,’ vol. xv, 1876, pp. 262—264), and by 
Mr. W. H. Hudleston (‘ Proc. Geol. Assoc.,’ vol. v, 1877, p. 443). The spicules 
were at first referred to perforate foraminifera, and named by Mr. Blake Renulina 
Sorbyana. As it is highly probable that these detached spicules may belong to 
more than a single species, I have preferred to give a particular designation to 
the forms described above. 
