INVERTEBRATE PALAEONTOLOGY 



speedy dissolution of very delicate tests. There are 

 plenty of Silurian limestones which might be expected 

 to afford complete preservation to calcareous Foramin- 

 ifera if they had been available. 



Among Radiolaria two suborders only, Spumellaria 

 and Nasselaria, are clearly recognized in the fossil 

 state. Of these, the former may with propriety be 

 considered the more primitive, since its members are 

 roughly spherical and have uniformly perforated 

 skeletons. The Nasselaria, with diverse form and 

 other specialized features, seem a more elaborate group, 

 It is in accord with expectation to find that Lower 

 Palaeozoic (and pre - Cambrian) Radiolaria are all 

 Spumellarians. They are rarely well-preserved (their 

 silica being usually devitrified into chert), and can be 

 recognized in thin rock-sections rather than collected 

 individually. 



(C) PORIFERA 



X Two groups of Sponges secrete skeletal matter ot 

 durability sufficient for fossilization. Of these, the 

 Silicispongiae tend to produce loosely associated spicules 

 which collapse after death ; Lithistids and Dictyonine 

 Hexactinellids alone have well-knit skeletons. The 

 Calcispongiae build relatiyehL_massiye structures, and 

 tn^ir^preference for coastal watersjgives them__betj 

 criances of preservation than the usually deep-water 

 Siliceous Sponges car^ attain^ 3N^y^rjh^lejsjQalca,reous 

 Sponges are unknown from _ the Lovver_ ^Palaeozoic, 

 whTIe compact jyrjgs ofSilicjspongiae occur in ajj 



formations from the Cambrian pnwards v , 



Detached spicules of Monactinellid sponges appear 

 first in the Silurian, but are rarely suited for precise 

 generic diagnosis. Lithistids representing all five sub- 

 orders occur in the Ordovician and Silurian; Tetra- 



