36 PORIFERA. 



the geological formations they are much less abundant 

 in argillaceous than in calcareous and arenaceous rocks. 

 Sponges are first found in the Cambrian System, the 

 earliest form being Protospongia from the Menevian Beds 

 and Lingula Flags. In the Tremadoc the Hexactinellid 

 Hyalostelia occurs, ranging onwards as far as the 

 'Chalk. In the Ordovician we have in the Llandeilo 

 Beds the first appearance of Ischadites associated with 

 Hyalostelia. In the Bala Beds we meet with Astylospon- 

 gia. The most abundant Silurian form is Ischadites; 

 Astrceospongia and Hyalostelia also occur. Sponges 

 are rare in the Devonian, but Sphcerospongia and Re- 

 ceptaculites have been recorded. In the Carboniferous 

 rocks, sponges become much more common, the siliceous 

 spicules often forming thick beds of chert : the Mon- 

 actinellids are represented by Reniera, the Tetractinellids 

 by Oeodites, the Lithistids by Doryderma, the Hexacti- 

 nellids by Hyalostelia, and the Heteractinellids by the 

 two component genera. Sponges appear to be absent in 

 the Permian and they are rare in the Trias, except in the 

 St Cassian Beds of the Tyrol, where Calcispongise are 

 numerous. In the Jurassic sponges are extremely abun- 

 dant; the only Monactinellid is Spongilla from the Purbeck 

 Beds; Lithistids and Hexactinellids although abundant 

 in Germany and Switzerland are rare in England; the 

 Calcispongiae are fairly numerous in this country as well as 

 in France and Germany. But of all the systems, it is the 

 Cretaceous in which sponges are most abundant. They are 

 found chiefly at four horizons: (1) in the Lower Greensand 

 of Faringdon, Upware, Kent,, and Surrey ; (2) in the Upper 

 Greensand and Chloritic Marl of Warminster, Blackdown, 

 Haldon, and the Isle of Wight ; the Lithistids being here ' 

 very abundant, exceeding the Hexactinellids; the Calci- ; 



