42 PORIFERA. DEMOSPONGI.E 



the Carboniferous Limestone, where they are represented 

 by the genera Geodites and Pachastrella. 



Order 5. Lithistida. The Lithistids have thick 

 stony walls and very variable external form. The spicules 

 (fig. 9, d, e) are stout and irregular in form, but sometimes 

 show four rays; the extremities branch or expand, and 

 by that means the spicules become firmly interlocked 

 with one another, but do not fuse together. These 

 irregular spicules (sometimes termed desmas) are formed 

 by secondary silica being deposited on small spicules of 

 the ordinary kind, which may be four-rayed or consist of 

 a single axis. In addition to these irregular spicules there 

 is generally a surface layer or cortex formed of trifid 

 spicules like those in the Tetractinellids. Flesh-spicules 

 are also present. Several different types of canal-system 

 occur. The Lithistids are closely allied to the Tetracti- 

 nellida, and are sometimes regarded as a division of that 

 Order. 



Owing to their solidity the Lithistids are preserved 

 abundantly as fossils. They are rare in the Palaeozoic; a 

 few are found in the Upper Cambrian of Canada ; in the 

 Ordovician and Silurian Astylospongia occurs; in the 

 Carboniferous Doryderma, etc. No forms belonging to 

 this Order have been found in the Devonian, Permian, or 

 Trias ; they are numerous in the Jurassic, attain their 

 maximum in the Cretaceous, and are scarce in the 

 Tertiary. 



Verruculina. Irregular, fan- or funnel-shaped, attached by 

 a short stalk. Oscula placed on prominent elevations on the upper, 

 and sometimes also on the under surface. Spicules small, inter- 

 lacing and forming a fibrous network. Ex. V. reussi, Upper Chalk. 



Pachinion. Cylindrical or club-shaped, tapering at its lower 

 part to a short stem. Central cavity large and deep, with vertical 



