ORDER III 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-lke branching ends 
of the rays, and the points of intersection (odes) 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 Rhizomorina,  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 an 
axial canal, and has extremities terminating in root-like strands or processes ; the 
spicules 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- Fic. 53. 
dina are usually svm- Aulocopium aurantium, Oswald. Diluvium; Sadowitz, Silesia. a, Indi- 
: sik tas vidual in 1/5 natural size ; b, Skeleton magnified 60 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 Silunian, 
are very scarce in the Upper Jurassic (Protetraclis), but common in the 
Cretaceous, Tertiary, and Recent periods. 
Aulocopium, 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- 

