ORDER IV SILIOISPONGIAE— HEXACTINELLIDA 59 



shaped, with short peduncle. Summit truncated or with shallow depression, 

 perforated by the postica of vertical excurrent canals. Exterior perforated by 

 ostia of the finer radial incurrent canals. Skeletal elements root-like, bent, 

 irregularly branching, with numerous short lateral processes. Upper 

 Cretaceous. /. polysloma (Roemer) ; /. punctata (Goldfuss). 



Chenendopora Lamx. (Fig. 62). Goblet-, funnel- or bowl-shaped, with 

 peduncle. Cloaca deep, perforated by postica of fine canals. Skeletal 

 elements numerously branched and containing branching axial canal. Upper 

 Cretaceous. 



Verrucidina Zitt. (Fig. 63). Foliate-, funnel-, ear- or bowl-shaped, short- 

 stemmed or sessile. Ostia on the upper surface surrounded by slight, collar- 

 like elevations. Middle (Gault) and Upper Cretaceous. 



Amphithelion Zitt. Like the preceding, but with both ostia and postica 

 terminating in bosses. Cretaceous. 



Other genera : Scytalia, Coelocorypha, Stachyspongia, Fachinion, Seliscothon 

 Zittel ; Megarliiza and Leiochonia Schrammen, etc., in the Middle and Upper 

 Cretaceous. 



Order 4. HEXACTINELLIDA O. Schmidt. 



(Triaxonia F. E. Schulze.) 



Siliceous sponges with six-rayed skeletal elements, the rays being normally disposed 

 in three axes intersecting at right angles, and containing axial canals ; elements either 

 detached or fused together so as to form a lattice-like mesh. Dermal and flesh spicules 

 exceedingly variable in form, but invariably six-rayed. 



Next to the Lithistida, the HexactinelUda are the most abundant of the fossil 

 siliceous sponges. They are extraordinarily variable in form, and are often 

 anchored by a tuft or " rope " of long, slender, vitreous fibres, or are attached 

 directly by the base. The walls are thin as a rule, and enclose usually a wide 

 cloaca ; the canal-system is consequently much simpler than in the Lithistida., 

 being made up merely of short tubes which penetrate the walls more or less 

 deeply on both sides, and generally end blindly. Sometimes the sponge is 

 entirely composed of thin-walled tubes which twine about one another irregu- 

 larly and produce a system of lacunar interstices (intercanals) of greater or 

 less size. 



The skeletal elements proper are distinguished by their considerably larger 

 size and simple form from the usually minute, astonishingly variable and 

 delicate flesh-spicules ; the latter, unfortunately, are seldom preserved in the 

 fossil state. The skeletal elements occur detached in the soft parts in the 

 Lyssacina group, or are partially or irregularly cemented together ; in the 

 Dictyonina group, on the other hand, they are regularly united in such manner 

 that the rays of proximate elements are all closely applied against one another, 

 and are surrounded by a continuous siliceous envelope. In this way a more 

 or less symmetrical lattice-work with cubical meshes is produced, in which, 

 however, the fusion of juxtaposed elements is indicated in that each ray con- 

 tains two distinct axial canals. The junction of the rays at the central node 

 of each element is usually inflated, but is sometimes sculptured in such manner 

 as to enclose a hollow octahedron (lantern nodes, lychnisks). The exterior of 

 the skeleton is often covered by a dermal layer composed of irregular hexactins. 



