34 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 



Gallery X. Hyalonematidse or Glass-rope Sponges, some of which are 

 Table-ease found fossil as for back as the Silurian. There are a number 

 Wall^case extinct families : Protospongidse (Cambrian and Silurian) ; 

 8b Sc. Dictyospongidae (Silurian and Devonian) ; Plectospongid^e 



(Ordovician and Silurian) ; Brachiospongida^ (Silurian) ; 

 Table-case PoUakidffi (Carboniferous and Cretaceous), and others. Here 



also have been placed the peculiar Receptaculitidse (Ordo- 

 Table-ease vician to Carboniferous), but it has been shown that their 



spicules were probably calcareous, and, alth(Dugh still 



.osc 



Fig. 10. — Reconstruction of Ventriculites, a Dictyonine Hexactinellid 

 sponge from the English Chalk, r, root-like processes of attachment ; 

 osc, osculum leading from the cloaca. A piece of the margin is broken 

 away to show the folds, which form the incurrent and excnrrent canals. 

 (After E. A. Minchin in E. Ray Lankester's " Treatise on Zoology." 

 By permission of the Editor.) 



exhibited with the sponges for the sake of convenience, they 

 are now not considered to be sponges at all. Some suppose 

 them to be calcareous algse. 



Sub-Class XL — DICTYONITsTA. The six-rayed spicules 

 of the middle layers of the body-wall overlap by their ends 

 and are then fused by a deposit of silica into a network, or 

 rather rafterwork, with square meshes (Greek dictyon, a net). 



Owing to their strong skeleton many Dictyonina are well 

 preserved as fossils, representing the following families : — 



