3G GUIDE TO THE FOSSIL INYERTEBEATE ANIMALS. 
Gallery X. typical .shapes other forms of spicule are associated (Fig. 8 h-rj). 
The spicules are generally arranged in radial bundles, but 
since they are 'not fixed together, the skeleton has generally 
fallen to pieces in course of fossilization. Thus only a few 
Table-case forms have been preserved entire, e.g. Pacliastrdia conmluta 
from the Upper Clialk of Flamborough (Fig. 11 a, 1). 
Detached spicules, on the other hand, are abundant, and 
form the main constituents of beds of sponge-rock in the 
Lower and Upper Greensand and in the Upper Chalk. 
Order II. — Lithistida. Branching secondary spicules 
(desmas), which may or may not be modified tet)‘axons, 
interlock to form a rigid skeleton (Figs. 12, 13) ; triaenes 
also may be present (Fig. 13 i). Owing to the firm manner 
Fig. 11. — Fos.sil sponge spicules: Silicisiiongiao, Tetractinellida Choristida. 
a and c are regular four-rayed caltrops ; i is a caltrop, with one ray 
suppressed ; I, a caltrop with two rays branching. The rest are triaenes 
of various shapes, d being one in which the shaft is reduced and the 
head-rays forked and stretched out flat, a and I belong to Pacha.itrella . 
a, g, j, enlarged 2fi diam. ; b, 40 diam. ; c, 54 diam. ; d, /, h,H, I, l.S 
diani. ; k, 100 diam.- (.After Hinde.) 
in which their skeletons are built up, Lithistida abound as 
Ibssils and are better represented in tlie Museum than even 
Table-cases the Hexactinellida. They are very diverse in form and size, 
13, 11. find their spicules also show great variety, some apparently 
Wall-cases pgjjig modified tetraxons, others inonaxons, and others poly- 
axons. It seems therefore that the Order has been derived 
from both Tetractinelliil and IMonactinellid ancestors, and 
its classification is naturally difficult, rending a satisfactory 
division into families, the following Sub-Orders are used by 
jialaeontologists : — Tetracladina (Cambrian to liecent) : 
desmas four-rayed with their ends jiroduced into twig-like 
])rocesses which interlock (Figs. 12 d, 13 c, d) ; Aidocopium, 
Ai 2 Jhonia (Fig. 10 e), and Jerca are well-known examples. 
