THE PORIFERA AND PROTOZOA 319 



other Palaeozoic genera, but from the Trias onwards a 

 continuous skeleton is found. Genera : Ventriculites (Cret., 

 Fig. 95, /, g), more or less inverted conical, with roots, 

 wall of cone folded ; Plocoscyphia (Cret.), frilled laminae 

 uniting into a coarse net-work ; Cceloptychium (Cret.), 

 mushroom-shaped, with folded wall. Siliceous sponges 

 attain their greatest abundance in Europe in the Creta- 

 ceous system. 



CLASS : CALCISPONGI^E. With calcareous 

 skeleton. There are several orders, with three-rayed 

 spicules, not forming a continuous skeleton, but little is 

 known of them in the fossil state. 



ORDER : Pharetrones. Spicules united into a fibrous 

 meshwork. Genera : Barroisia (Cret), clusters of double 

 cylinders, the space between the inner and outer cylinder 

 being crossed by partitions much coarser than the tabulae 

 of a coral ; Peronidella (Trias. -Cret.), hollow, cylindrical ; 

 Raphidonema (Cret.), more or less cup-shaped. Calcareous 

 sponges are locally very abundant in the Upper Jurassic 

 of South Germany, the Lower Cretaceous of Faringdon 

 in Wiltshire and Upware in Cambridgeshire, and the 

 Upper Cretaceous (Cenomanian) of Westphalia, but are 

 rare in general. 



The Protozoa are animals composed of a single cell, 

 or a small number of undifferentiated cells. Only two 

 orders leave fossil remains the Radiolaria, which form 

 a beautiful lattice-work skeleton of silica, and the 

 Foraminifera, which have one of calcite, aragonite, or of 

 agglutinated foreign bodies. The majority of forms in 

 both orders are very small, ranging from i mm. diameter 

 downwards, but some of them occur in such abundance 

 at certain horizons as to be quite important rock-forming 



