32 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 



Gallery X. by the Jurassic Protosycon. The diminutive Leucandra 

 Walfordi from the Middle Lias is the sole fossil example 

 of the common recent family Grantidae, for in them the 

 spicules are loosely and irregularly distributed chiefly around 

 the chambers. 



The Pharetronidae have a skeleton of fibres formed by 

 spicules arranged side by side and interlocking, but not fused 

 (except in the Lithoninae). In some a relatively large three- 

 rayed or four-rayed spicule is enveloped by smaller thread- 

 like spicules ; in others the spicules are approximately 

 equal throughout. In many there is an outer layer formed 

 of a close felt of spicules. The more important genera are 

 Table-cases Corynella (Trias to Cretaceous), Holcospongia (Jurassic), Elas- 

 WaU-eases Peronidella (Jurassic and Cretaceous), and 



8b, 7b, 7c. Pharetrospongia (Cretaceous). All these belong to the 

 Sub-family Dialytinae. The Lithoninae, in which the main 

 spicules are fused, contain a few Tertiary and Cretaceous 

 Table-case genera, of which the Chalk Porosphaera is the best known. ' 



The structure of the skeleton has often been greatly 

 altered in fossilization. The spicules have lost their out- 

 lines, and the fibres now appear as if entirely formed of 

 granular or fibrous calcite. In other cases the fibres have 

 been replaced by silica, so that they remain after treatment 

 with acid, but all trace of spicules has been obliterated. 

 Table-case In some specimens of Pharetrospongia, preserved in solid 

 flint, the outer portion of the fibres has been replaced by 

 silica, whilst their interior still retains the original calcite. 

 The structure even in the best preserved specimens is hardly 

 recognisable, unless in thin sections under the microscope. 



The Calcarea of to-day are marine shallow- water forms ; 

 but some Pharetrones may have lived in deeper water. 



Branch : SILICISPONGIAE. 



Class IL— HEXACTINELLIDA. 



Skeleton of siliceous spicules (Fig. 9), all triaxon and 

 therefore primitively six-rayed (hexactine). Canal-system 

 simple, with thimble-shaped chambers. The body-wall, 

 which is relatively thin but may be thickened by folding, 

 surrounds a wide funnel-shaped or cylindrical cavity (cloaca) 

 into which the chambers discharge their currents (Fig. 10). 

 In this wall the chambers are suspended by rafters (trabe- 

 cule) of soft tissue between an outer dermal membrane and 

 an inner gastral membrane. The spicules are formed by cells 



