M. K. A. Zittel on Fossil LithisiidcB. 127 



nallj siliceous skeleton into calc-spar, in general no consider- 

 able change takes place in the form of the small skeletal parts. 

 For example, if we cut a Cnemidiastrum or a Hyalotragos 

 from the Swabian Jura at any point and examine the cut 

 surface with the lens or under the microscope by direct light, 

 the somewhat dark-coloured skeletal corpuscles, consisting of 

 calc-spar, stand out sharply from the lighter rock-mass which 

 has penetrated the sponge, and the structure may thus be 

 recognized without further preparation. With a little practice 

 mere examination with the lens, or, under certain circum- 

 stances, even with the naked eye, will suffice for the immediate 

 recognition of the different genera of Hexactinellidae and 

 LithistidfB. 



Carter distinguishes three kinds of characteristic siliceous 

 structures in the skeleton of the Lithistidee : — 



1. The true " skeleton-spicules " interwoven by means of 

 sarcode and of thin filigree-branched ends ; 



2. The " surface-spicules," which are generally furnished 

 with a vertical shaft; and 



3. The so-called " flesh-spicules," uniaxial siliceous cor- 

 puscles, usually of small size, which lie freely in the sarcode, 

 generally in the greatest abundance at the surface of the sponge- 

 body, but also occur more or less frequently in the interior. 



Of these three constituents, the small " flesh-spicules"* are 

 deficient in all fossil Lithistidce : but even in living specimens 

 they can only be observed when the skeletons are still fur- 

 nished with their sarcode coating ; when the latter has been 

 removed by decomposition, these minute corpuscles disappear 

 with it. 



According to the recent investigations of Sollas, the flesh- 

 spicules are rapidly destroyed by alkalies ; and therefore they 

 could not long resist the process of fossilization. 



Leaving out of consideration these little flesh-spicules, many 

 Lithistidse are further provided with large uniaxial bacillar 

 spicules or cylinders, which also lie in great quantities at the 

 surface or in the canals, and sometimes form a complete 

 spicular coat. These large bacillar spicules appear, in certain 

 fossil Lithistidse, to replace the anchor-sliaped surface-spicules, 

 and may therefore be more particularly considered hereafter 

 with them. 



The denomination spicules does not apply very well to the 



* Good figures of these little flesh-spicules are to be found in Bower- 

 bank's " Monograph of the Siliceous Sponges," P. Z. S. 1869, pi. v. figs. 7 

 & 8, pi. vi. figs. 8 & 10-14, and pi. xxiii. fig. 6, 



