ORDER in SILICISPONGIAE LITHISTIDA 47 



united by zygosis. Symmetrical, tetraxial, uniaxial, or polyaxile dermal and flesh- 

 spicules also present. 



The Lithistids are closely related to the Tetractinellids, and, in the opinion 

 of many zoologists, constitute with them but a single order. 



The Lithistids are peculiarly well suited for preservation, owing to the 

 massive, stony character of their skeletons ; and their remains occasionally form 

 thick deposits, especially in the Jurassic and Cretaceous. Their outer configu- 

 ration is extremely variable ; most commonly it is bowl-shaped, cup-shaped, 

 pyriform, globular, bulbous, or plate-like ; while the body is attached either by 

 the base or by means of a peduncle. The canal-system varies greatly in different 

 genera, but is usually well developed and more or less complicated. The four- 

 rayed skeletal elements are interlocked by means of the root-like branching ends 

 of the rays, and the points of intersection (nodes) with the ends of adjacent 

 uniaxial spicules are thickened into balls. The uniaxial, usually very irregular 

 skeletal elements are interlaced on all sides by means of root-like processes. 

 Dermal and flesh-spicules are preserved only under exceptionally favourable 

 conditions, but are invariably present in recent genera, and furnish valuable 

 systematic characters. The classification of fossil Lithistids is based wholly 

 upon the skeletal elements and canal -systems. Five principal groups are 

 recognised, whose subdivision into families need not concern us at present : 

 Tetracladina, Eutaxicladina, Anomocladina, Megamorina, and Ehizomorina. Exist- 

 ing Lithistids occur most abundantly at depths ranging between 100 and 400 

 metres, but are occasionally found as deep as 1800 metres. 



Sub-Order A. TETRACLADINA. Zittel. 



Skeletal elements composed of four usually equal rays, each of which encloses 

 axial canal, and has extremities terminating in root-like strands or processes; 

 spicides are intertwined 

 to form an open mesh- 

 work. Dermal spicules 

 either grapnel - like te- 

 traxons, frequently with 

 furcate prongs, or dis- 

 coidal with entire or 

 lobate margin; or they 

 are nail - shaped or 

 , cylindrical monaxons. 



The skeletal ele- 

 ments of the Tetracla- FIG. 53. 



dina are Usually SVm- Aulocopium auvuitium, Oswald. Diluvium; Sadowitz, Silesia, a, Indi- 

 , . , vidual in !/.-> natural size ; 6, Skeleton magnified GO diameters. 



metrical tetraxons, 



whose four smooth, more rarely tuberculate or knotty rays intersect approxi- 

 mately at an angle of 109. They occur in the Cambrian and Silurian, 

 are very scarce in the Upper Jurassic (Protetraclis), but common in the 

 Cretaceous, Tertiary, and Recent periods. 



Aidocopium, Oswald (Fig. 53). Hemispherical or bowl-shaped with short 

 peduncle ; inferior surface covered with dense, wrinkled, silicious skin. Cloaca 

 central ; sponge body with numerous arched canals parallel to contour of peri- 



