FIG. 6. Sponge spicules (skeletal), a, monactinellid, Halichondria 

 panicea, Kecent. b, tetractinellid, Pachastrella sp. Upper Green- 

 sand, c, tetractinellid, Geodites sp. Eocene, d, lithistid, Scytalia 

 radiciformis, Chalk, e, lithistid, Seliscothon mantelli. f, hexactinellid 

 Cceloptychium agaricoides, Chalk, g, octactinellid, Astrceospongia 

 sp. Silurian, h, heteractinellid, Asteractinella expansa, Carbo- 

 niferous. .;', calcisponge, Grantia compressa, Kecent. All magnified. 



