188 BRITISH PALEOZOIC SPONGES. 



seven Tetractinellids, three Lithistids, five Hexactinellids, six Heteractinellids, 

 and one species of Calcisponge. No Sponge has been discovered in Permian 

 strata. 



Of the total number of species, forty-three are limited to a single system ; one 

 species passes from the Cambrian to the Ordovician, two from the Ordovician to 

 the Silurian, one from the Ordovician to the Carboniferous, and one from the 

 Silurian to the Devonian. Two of the Carboniferous species are, so far as can be 

 determined from the spicules, identical with forms from the Lower Cretaceous of 

 the South of England. 



Only ten out of the fifty species are known to occur beyond the British area. 

 These forms are present at corresponding geological horizons in Scandinavia, 

 Germany, Belgium, Russia, and North America. 



Of the species hitherto included as Palasozoic Sponges, twenty-eight are removed 

 as bad or doubtful forms. 



