82 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 
Gallery X. by the Jurassic Protosycon. The diminutive Lcucandra 
Walfordi from the jMiddle Lias is the sole fossil example 
of the common recent family Grantidae, for in them tlie 
spicules are loosely and irregularly distributed chiefly around 
the chambers. 
The Pharetronidac 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), (Jurassic), .£'/as- 
Wall-eases Peronidclla (Jurassic and Cretaceous), and 
8b, 7b, 7o. Pharetrosponyia (Cretaceous). All these belong to the 
Sub-family Dialytinae. The Lithoninae, in which the main 
spicules are fused, contain a few Tertiary and Cretaceou.s 
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been replaced by silica, so that they remain after treatment 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 Pharetrosponyia, 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flint, the outer portion of the fibres has been replaced by 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 II.— 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 cun’ents (Fig. 10). 
In this wall the chambers are suspended l.)y rafters (trabe- 
culae) of soft tissue between an outer dermal membrane and 
an inner gastral membrane. The spicules are formed by cells 
