318 PAL/EONTOLOGY 



into a meshwork, and in such cases the form of the body 

 is preserved in the fossil. The substance never persists 

 as unaltered opal ; it may simply change into chalcedony, 

 or may be replaced by pyrites, marcasite (passing into 

 limonite), or calcite. 



The common bath-sponge belongs to a class having a 

 continuous skeleton of a horny organic substance, but 

 such forms are not preserved as fossils, and palaeontologists 

 are only concerned with two classes, distinguished by. the 

 chemical composition of the skeleton. 



CLASS : SILICISPONGLE.* Skeleton of silica. 



ORDER i : Monaxonida. Spicules one-rayed (Fig. 

 95, a). Only known fossil by loose spicules, except one 

 genus Cliona, a boring sponge in lamellibranch shells 

 and belemnite-guards, known by its borings, often 

 preserved as internal casts in flint. 



ORDER 2 : Tetraxonida. Spicules four-rayed (Fig. 

 95 b, c). 



SUB-ORDER : CHORISTIDA. Spicules not united. 



SUB-ORDER : LITHISTIDA. Spicules united into a con- 

 tinuous skeleton (Fig. 95, c). Genera: Siphonia (Cret., 

 Fig. 95, d), pear-shaped, stalked; Doryderma (Cret.), 

 cylindrical, branched ; Verruculina (Cret. ), cup-shaped or 

 irregular, with short stalk. 



ORDER 3 : Hexactinellida. Spicules of six rays, 

 intersecting at right angles (Fig. 95, e). In the earliest 

 genus Pvotospongia (Camb.) the spicules were very feebly 

 united, as they are found only a few together, the form 

 of the body being unknown. The same is the case with 



* Zoologists do not admit this as a natural group: they make 

 Hexactinellida a class in itself, and unite the remainder with the 

 horny sponges as a class, Demospongiae. 



