158 INVERTEBRATE PALAEONTOLOGY 



species of such size that they can be readily seen with the 

 naked eye, and may even be separated from the matrix. 



Devonian Foraminifera are hardly superior to those of 

 the Silurian in abundance or variety ; and, in view of the 

 obscure condition of British strata of this type, can be 

 ignored. But in Carboniferous times the order rose to a 

 position of faunal importance. Plentiful development of 

 more or less open-sea limestones may account, in some 

 measure, for the abundance of Foraminifera, but 

 evolutional progress in the secretion of durable tests 

 is probably a contributory cause. Nowhere in Britain 

 do the Carboniferous strata show the characters of 

 Foraminiferal ooze, but the less pure parts of the 

 Mountain Limestone of Northern counties are locally 

 rich in the " arenaceous " form Saccammina. A com- 

 parable part is played by Fusulina (a member of the 

 Nummulite family) in Upper Carboniferous Limestones 

 in Eastern Europe. The long-lived group of the Textu- 

 lariidae is well represented, especially by forms with 

 "agglutinating" habit. Permian Foraminifera corre- 

 spond very closely with those of the preceding period, 

 but they are naturally very scarce in Britain. 



Among Radiolaria, the Spumellaria still predominated 

 in the Upper Palaeozoic ; the more complex Nasselaria 

 occur in relatively small, though far from negligible, 

 numbers. 



(C) PORIFERA 



Important additions to the Porifera were initiated in 

 the Upper Palaeozoic. All the types found in the older 

 series continued (Receptaculitidae disappearing in the 

 Carboniferous), while siliceous Tetractinellids, and the 

 Calcispongiae, made their first appearance. With the 

 exception of the Hexactinellid Hyalostelia, sponges are 

 usually rare in British Devonian and Carboniferous rocks. 



