120 Guide to the Invertelrata. 



Sponges. The Lyssaklne division includes a number of forms mostly from 

 GALLEBY Palaeozoic rocks, which are but very imperfectly known owing to 

 West Side ^^^^'"" fragmentary condition. They have been placed in the families 

 Wall-cases of the Protospongidse, Dictyospongidse, and Plectospongidae. 

 7*8, The Dictyonine Hexactinellids comprise a very large number 



cases of fossil forms, whose preservation is due to the manner in 



11 15. which the spicular mesh has been fused into a connected frame- 

 work. They have been divided into the following families : — 

 (a) Craticularidse. Mostly cup- or funnel-shaped ; spicular nodes 

 simple and not perforate. Canal apertures large, simple, ending 

 blindly in the skeleton, (h) Coscinoporidae. In addition to cup- 

 and funnel-shaped forms, many sponges of this family have thin 

 walls, folded into a series of flanges, as in the Cretaceous genus 

 Guettardia. The surface canal apertures are small, and usually 

 regularly arranged in quincunx, {c) Staurodermidae. Usually 

 funnel-shaped or cylindrical sponges, with an irregular skeletal 

 mesh, and a very definite dermal layer in which large cruciform 

 spicules are imbedded, {d) Yentriculitidae. Mostly funnel-shaped 

 sponges in which a thin wall is folded vertically and usually 

 radially arranged. The spicular nodes are hollow or lantern- 

 shaped, and there is a modified dermal layer. The base of the 

 sponge has root-like extensions of spicular fibres. Ventriculites, 

 Mant., is numerously represented in the Upper Chalk, {e) Coelo- 

 ptychidae. The sponges are mushroom-shaped, and they consist 

 of a thin wall arranged in radial folds and inclosed in a perforate 

 siliceous dermal layer. Canal apertures are in rows on the ridges 

 of the under-surface. The spicular mesh with lantern nodes. 

 (/) Maeandrospongidse. These sponges are pear- or sack- shaped, or 

 in irregular nodose masses, which consist of numerous anastomosing 

 folds of a thin wall of delicate spicular meshwork. These folds 

 are in some forms either partially or entirely inclosed in a case of 

 fine spicular membrane, as in the genera Camerospongia, D'Orb., 

 and Cystispongia, Eomer. To this family belongs also Floco- 

 scyphia, Keuss, which is common in the Upper Chalk. 



Hexactinellid sponges contrast generally with the preceding 

 group of Lithistids in having thin, delicate walls surrounding wide, 

 funnel-shaped, or cylindrical, cloacal cavities. In those instances 

 in which an apparently thick wall forms the body of the sponge, 

 it IS usually found to really consist of numerous convolutions of 

 a simple thin wall of spicular tissue. 



