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



If we now return to the simpler, short-stalked, furcate an- 

 chors with curved arms of Theonella, we find that these are 

 approached by other more complicated forms. Thus the shaft 

 is reduced to a short conical style, and the curved arms emit 

 lateral branchlets, which, in their turn, are beset with root-like 

 excrescences. Elegant structures are thus produced (see Bow- 

 erbank, Proc. Zool. Soc. 1869, pi. v. figs. 2-4, and pi. xxv. 

 fig. 4) , which in their appearance considerably approach the true 

 skeletal corpuscles. This resemblance becomes still greater 

 when the short shaft itself runs out at its extremity into fili- 

 greed processes {Azo7-ica Pfeifferce, Cart.). 



In these last-mentioned " surface-spicules," their relation 

 to the skeletal elements is quite indubitable ; but there are 

 many, especially fossil, Lithistidse in which the siliceous cor- 

 puscles of the surface, although differing in size and ramifica- 

 tion from those of the rest of the skeleton, can only be regarded 

 as modified skeletal corpuscles, but cannot be referred back to an 

 anchorlike structure [Leiodermatium^ LeiodoreUa^ Verrucu- 

 Una, Amphithelion, SeliscotJion, ChonelJa, &c.). I regard 

 such " surface-spicules " merely as young still undeveloped 

 skeletal elements. 



The arrangement of the anchor-shaped surface-spicules is 

 almost invariably such that the shaft is turned inwards and the 

 prongs outwards. In Corallistes, Turonia, Gallo'pegma^ Calym- 

 matina, Theonella, &c. the double prongs of the anchors, 

 which diverge in the same plane, form a remarkably elegant 

 stellate pavement, the interstices of which were occupied in 

 the living state by sarcode and minute flesh-spicules. In 

 Doryderma the anchors, which are furnished with short double 

 prongs, are remarkable for the considerable length of their 

 shafts. They are grouped together in dense tufts, and stick, 

 with their notched ends outwards, in mesh-like depressions of 

 the skeleton. The lobate and notched short-shafted anchors 

 and the siliceous disks of Discodermia &c. also form a more or 

 less dense surface-layer, which is the more perishable in pro- 

 portion as the shafts, which are directed inwards, penetrate to 

 a less distance into the mass of the skeleton. 



As a rule, those surface-structures which, in their general 

 habit, differ least from the true skeletal corpuscles, and are 

 probably only young undeveloped skeletal elements, are 

 placed very close together. Sometimes they form an appa- 

 rently solid and smooth siliceous membrane, which either covers 

 only certain portions of the sponge-body {Turonia, Chenen- 

 dopora, Thecosiphonia) ., or else clothes the whole sponge as a 

 regular fine siliceous envelope [Galymmatina, Astrocladia) . 

 D'Orbigny, Fromentel, Courtiller, and Pomel have repeatedly 



