48 PORIFERA 



Hexactinellids by Hyalostelia, and the Heteractinellids by 

 Tholiasterella and Asteractinella. 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 the 

 Calcarea are numerous. 



In the Jurassic, sponges are extremely abundant ; the 

 only Monaxonid is Spongilla from the Purbeck Beds ; 

 Lithistids and Hexactinellids although common in Ger- 

 many and Switzerland are comparatively rare in England ; 

 the first group is represented by Platychonia, the second 

 by Craticularia, Verrucoccelia, etc. ; the Calcarea are 

 numerous in this country as well as in France and Ger- 

 many, common genera being Peronidella, Corynella, and 

 Holcospongia. The occurrence of Hexactinellids in the 

 Inferior Oolite is noteworthy, since other evidence shows 

 that that deposit was laid down in shallow water, but at 

 the present day Hexactinellids are characteristic of deep 

 water. 



Sponges are more abundant in the Cretaceous than in 

 any other system ; in England they are found chiefly at 

 four horizons : — (1) in the Lower Greensand of Faringdon, 

 Up ware, Kent, and Surrey, where the Calcarea are much 

 better represented than the other groups, Peronidella, 

 Barroisia, and Rhaphidonema being common forms : 

 (2) in the Upper Greensand and Chloritic Marl of War- 

 minster, Blackdown, Haldon, and the Isle of Wight, where 

 the Lithistids {e.g. Dory derma, Siphonia, Hallirhoa) are 

 very abundant, exceeding the Hexactinellids {e.g. Craticu- 

 laria, Plocoscyphia, Stauronema) ; the Calcarea are also 

 common in places : (3) in the Lower Chalk of the south 

 of England, where we find Siphonia, Craticularia, Stauro- 

 nema, Plocoscyphia, etc. ; the Calcarea are rare : (4) in 

 the Upper Chalk, where the siliceous sponges are very 



