M. K. A. Zittel on Fossil Lithistidce. 123 



thick wall of the cup-shaped, basin-shaped, top-shaped, or 

 cylindrical sponge-body consists of vertical laminae of small 

 thickness, or of wedge-shaped segments, separated from each 

 other by perpendicular clefts, which are either simple or 

 divided towards the outside. By this means the whole sponge 

 acquires a decidedly radiate structure, and in many cases re- 

 minds one of the calice of a coral with numerous radiating 

 septa. The vertical clefts are bridged over at certain regular 

 distances by skeletal layers, which consequently divide each 

 cleft into a complete system of parallel radial canals standing 

 one above the other. The latter penetrate the wall and open 

 at the outer surface and on the wall of the central cavity in 

 rounded or irregular pores. Striking examples of this form 

 of the canal-system are furnished by the genera Cnemidias- 

 trumj Corallidium, and Seliscothon. 



Finally it may be mentioned that veiy frequently, at the 

 surface where the growth of the sponge takes place, therefore 

 especially at the vertex, the canals in course of formation 

 appear as radiating furrows of very various nature, and up to 

 a certain point indicate the course of the canal-system in the 

 whole sponge-body. 



Condition of the Sheleton and State of Preservation. 



The skeleton of the Lithistidse is remarkable for its stony, 

 solid texture. The sarcode sinks into the background rela- 

 tively to the siliceous deposits, and in living forms exists only 

 in comparatively small quantity. As, moreover, the walls 

 (or indeed the whole sponge-body) are of considerable thickness 

 and usually traversed only by comparatively fine canals, the 

 Lithistidas must be reckoned among the most persistent and 

 resistant of sponges. It is true that the small skeletal ele- 

 ments do not fuse together, as in the Hexactinellidge, to form 

 a coherent framework ; but they are so closely interlocked that 

 even after the death of the animal they do not fall asunder, so 

 as to be scattered by the waves like the spicules of other 

 siliceous sponges. This stony texture of the Lithistidai speci- 

 ally adapts them for preservation in the strata of the earth ; 

 in fact a great proportion of the old Petrospongise belong to 

 this group. Well-preserved skeletons, freed from matrix by 

 muriatic acid, are scarcely distinguishable in their appearance 

 and texture from the bodies of recent forms freshly macerated 

 or newly taken from the sea. 



There are certain localities, especially in the Upper Cre- 

 taceous of North Germany (Ahlten, Lemforde, and Linden in 

 Hanover ; Vordorf and Biewende in Brunswick ; Coesfeld, 

 Legden, and Darup in Westphalia), where the fossil Lithis- 



