TORIFERA 47 



numbers beyond that limit. The Tetractinellids are also 

 common in water of less depth than 200 fathoms, but 

 extend down to 2000 fathoms. The Lithistids range from 

 7J to 1075 fathoms, and are most abundant between 100 

 and 150 fathoms. The Hexactinellids occur in deeper 

 water than the Lithistids, being found down to a depth 

 of 2900 fathoms ; but they are abundant between 100 and 

 200 fathoms, and again between 300 and 700 fathoms. 

 The Calcarea are mainly shallow water forms. 



The fossil forms are comparatively rare in the Palaeozoic 

 rocks until we reach the Carboniferous; and throughout 

 the geological formations they are much less abundant 

 in argillaceous than in calcareous and arenaceous rocks. 

 Sponges are first found in the Lower Cambrian rocks ; the 

 earliest British form is Protospongia from the Menevian 

 Beds and Lingula Flags ; in the Tremadoc the Hexacti- 

 nellid genus Hyalostelia occurs, ranging onwards as far as 

 the Chalk. In the Ordovician we have in the Llandeilo 

 Beds the first appearance of Ischadites 1 , associated with 

 Hyalostelia ; in the Bala Beds we meet with Astylospongia. 

 The most abundant Silurian form is Ischadites; Astraso- 

 spongia, Phormosella, and Hyalostelia also occur. Sponges 

 are rare in the Devonian, but Astrceospongia, Sphcero- 

 spongia\ and Receptaculites 1 have been recorded. In the 

 Carboniferous rocks, sponges become much more common, 

 the siliceous spicules often forming thick beds of chert : 

 the Monaxonids are represented by Reniera, the Tetracti- 

 nellids by Geodites, the Lithistids by Doryderma, the 



1 The sponge-character of the Silurian and Devonian genera Ischadites, 

 Receptaculites, and Sphcerospongia, which have been placed by some 

 authors in the Hexactinellida, is now disputed ; if they are sponges it is 

 probable that they belong to the Calcarea, since their skeleton appears to 

 have consisted originally of carbonate of lime. 



