POEIFEEA (sponges). 39 
right angles to this plane. The Silurian Astraeospongia is 
the only genus. 
Class V. (?) HETERACTINELLIDA, 
Silicispongiae, in which the normal spicule is large, 
with from six to thiity rays, radiating variously from a 
common centre (Fig. 8 a). Contains only the Carboniferous 
Tholiaderdla and Aster actindla. 
Although the spicules are aberrant, it is often maintained 
that the Octactinellida and Heteractinellida should be placed 
with the Hexactinellida. 
Turning now to the Table-cases with the British 
fossil Sponges, we find some obscnre fragments from the 
Cambrian Durness Limestone of Scotland, and then the 
oldest known sponge, Protospongia fenestrata, from the 
Cambrian rocks of St. Da\dds, S. Wales. All that is 
preserved are fragments of the mesh work, in which the 
silica is now replaced by pyrites. They show, however, 
very primitive triaxon spicules, which enable one to refer 
the genus to the Hexactinellida Lyssacina. Here also 
are placed the Silurian Amphispongia, Plectoderma, and 
Hyalostdia. A specimen of the last shows a root-tuft. 
The Lower Carboniferous rocks of Yorkshire and Ayrshire 
have yielded large root-tufts and clearly preserved body- 
spicules of Hyalostdia. Among the other spicules here 
shown, special note should be taken of the Heteractinellid 
genera, Asteradindla and Tholiasterdla. Lithistid, Tetracti- 
nellid, and Monactinellid spicules from 
Ayrshire, and a Calcisponge from Fife 
are also exhibited. 
The Calcispongiae first appear in 
force in a series from tlie Inferior Oolite, 
Great Oolite, and Corallian ; they form 
ma.'^sive or stout branching stocks. The 
marine Jurassic rocks have also yielded 
Hexactinellid, Tetractinellid, and Lith- 
istid sponges, while from the Purbeck 
beds comes a freshwater Spongillo. to 
represent the IMonactinellida. 
Among Cretaceous rocks the Lower Greensand of Faring- 
don, Berks, has long been famous for its beautifully 
preserved Calcisponges, of which a fine series is shown. The 
chert beds of this age in Kent and Surrey are mainly com- 
Fig. 15. — A Calcisponge, 
Peronidella pistilli- 
formis, from Great 
Oolite near Bath. 
^ nat. size. 
Gallery X. 
Wall-case 
8b. 
Table-case 
15. 
Table-case 
15. 
Table-case 
15. 
Table-case 
14. 
Table-case 
14. 
