36 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery X. typical shapes other forms of spicule are associated (Fig. 8 h-g). 



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. Pachastrella convohUa 

 from the Upper Chalk of Flamborough (Fig. 11 a, I). 

 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 tetraxons, 

 interlock to form a rigid skeleton (Figs. 12, 13) ; triaenes 

 also may be present (Fig. 13 ^). Owing to the firm manner 



Fig. 11. — Fossil sponge spicules : Silicispongiae, Tetractinellida Choristida. 

 a and c are regular four-rayed caltrops ; 6 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 Pachastrella. 

 a, g, j, enlarged 26 diam. ; b, 40 diam. ; c, 54 diam, ; d, f, h, i, I, 13 

 diam. ; k, 100 diam. (After Hinde.) 



in which their skeletons are built up, Lithistida abound as 

 fossils and are better represented in the Museum than even 

 Table-eases the Hexactinellida. They are very diverse in form and size, 

 13, 11. and their spicules also show great variety, some apparently 

 being modified tetraxons, others monaxons, and others poly- 

 axons. It seems therefore that the Order has been derived 

 from both Tetractinellid and Monactinellid ancestors, and 

 its classification is naturally difficult. Pending a satisfactory 

 division into families, the following Sub- Orders are used by 

 palaeontologists : — Tetracladina (Cambrian to Eecent) : 

 desmas four-rayed with their ends produced into twig-like 

 processes which interlock (Figs. 12 d, 13 c, d) ; Aidocopmm, 

 Si'plionia (Fig. 16 c), and Jerea are well-known examples. 



Wall-cases 

 7, 8. 



