54 COELENTERATA—SPONGIAE CLASS I 
Amphithelion, Zitt. Like the preceding, but with both ostia and postica 
terminating in bosses. Cretaceous. 
Other genera: Scytalia, Coelocorypha, Stachyspongia, Pachinion, Séliscottne Zitt. 
etc., in the Middle and Upper Cretaceous. 
Order 4. HEXACTINELLIDA. O. Schmidt. 
(Triaxonia, F. E. Schulze.) 
Silicious sponges with siv-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 siz-rayed. ; 
Next to the Lithistida, the Hexactinellida are the most abundant of the fossil 
silicious sponges. They are extraordinarily variable in form, and are often 
anchored by a tuft or “rope” of long, s slender, glassy 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 Lithistidu, 
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 irregularly 
and produce a system of lacunar interstices (intercanals) of greater or lesser size. 
The skeletal elements proper are distinguished by their considerably larger 
size and uniform type of structure, from the usually minute, astonishingly 
variable and delicate flesh-spicules ; the latter, unfortunately, are very seldom 
preserved in the fossil state. The skeletal elements occur detached in the soft 
parts in the Lyssacina group, or they are but partially or irregularly cemented 
together ; in the Dictyonina group, on the other hand, the skeletal elements 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 silicious 
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 by the fact that each ray contains two distinctly separated 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, in which the externally 
directed ray has become atrophied ; or a dense silicious envelope is secreted, in 
which stellate hexactins with reduced outwardly and inwardly directed rays 
(stauractins) are embedded in greater or lesser profusion. 
The Hexactinellida of the present day are distributed chiefly over the greater 
depths of the ocean beyond the hundred fathom line (200 to 3000 fathoms). 
They occur fossil principally in deep-sea deposits, and make their first appear- 
ance in the Cambrian; their period of greatest development falls in Jurassic 
and Cretaceous time. 
Sub-Order A. LYSSACINA. Zittel. 
Skeletal elements either entirely detached, or only partially and in an irregular 
Fashion cemented together. oot-tuft often present. 
