188 BRITISH PALAJOZOIC 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 Paleozoic Sponges, twenty-eight are removed 
as bad or doubtful forms. 
