POEIFEEA (sponges). 



39 



right angles to this plane, 

 the only genus. 



The Silurian Astraeospongia is Gallery X. 



Wall-ease 

 8b. 



Class Y. (?) HETERACTINELLIDA. 



Silicispongiae, in which the normal spicule is large, 

 with from six to thirty rays, radiating variously from a 

 common centre (Fig. 8 a). Contains only the Carboniferous Table-case 

 Tholictderella and Aster actinella. 



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 first the oldest known sponge, Protosimigia Table-case 



fenestrata, irom the Cambrian rocks of St. Davids, S. Wales. 

 All that is preserved are fragments of the meshwork, 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 Am])hispongia, Plectodcrma, and 

 Hyalostelia. A specimen of the last shows a root-tuft. 



Devonian sponges are represented only by the strange 

 Lodanella, which certainly has the form of a simple type of 

 sponge, presumably one of the Calcarea, since no spicules are 

 preserved. 



The Lower Carboniferous rocks of Yorkshire and Ayrshire 

 have yielded large root-tufts and clearly preserved body 

 spicules of Hyalostelia. Among the other spicules here 

 shown, special note should be taken of the Heteractinellid 



15. 



genera, Asteractmella and TJioliasterella. 

 nellid, and Monactinellid spicules from 

 Ayrshire, and a Calcisponge from Fife 

 are also exhibited. 



The Calcispongiae first appear in 

 force in a series from the Inferior Oolite, 

 Great Oolite, and Corallian ; they form 

 massive 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 Spongilla to 

 represent the Monactinellida. 



Among Cretaceous rocks the Lower 

 don, Berks, has long been famous 



Lithistid, Tetracti- 



Fig. 15. — A Calcisponge, 

 Peronidclla pistilli- 

 forniis, from Great 

 Oolite near Bath. 



Greensand of Faring- 

 for its beautifully 



Table-case 

 15. 



Table-case 

 15. 



Table-ease 

 14. 



Table-case 

 14. 



