30 Die Sihtrische Fauna des Westlichen Tennessee. 



species, very numerous, but still undescribed, which agrees with a 

 species of Sadewitz. In general, all the Silurian sponges that haVe yet 

 been found seem to belong to the geognostic level of the limestone o^ 

 tlie Island of Gothland. Up to this time no sponges, that can with cer- 

 tainty be determined, have been found in the lower division of the 

 Silurian Group. 



That, in spite of the calcareous nature of the strata, the petrified mass 

 is always siliceous can evidently be not accidental, but certainly de- 

 pends upon the essentially siliceous constitution of the corporeal sub- 

 stance of the animal during life. [The substance of living sponges is 

 also in great part siliceous. I have received through Prof. Steensti'up, 

 in Copenhagen, pieces of the well known, large, goblet-formed spongke 

 vesparia, Lam., in which all organic substance has been destroyed by 

 means of white heat. The form and structure of the sponge has re- 

 mained entirely unchanged, which can only be explained by the foct, 

 that, according to an analysis by Forchammer, the species contains in 

 its fresh condition seventy-five per cent, of silica.] By means of this 

 physical constitution other silica, dissolved in water, has been attracted 

 in the process of petrifaction, and the hollow space of the texture 

 filled up by it just as during the life of the animal; the calcareous na- 

 ture of the shell and spines of the Echinidce causes the compact sparry 

 calcareous constitution of the fossil Echinidce. It is certain that be- 

 tween the living and the fossil sponges there is no such important and 

 thoroughgoing difference in the compactness of the texture, as D'Or- 

 bigny (Cours ele.nent de Pal. et Geol., 209), and after him Pictet 

 (Traite de Paleontol., e»l. 2, torn, iv., 530), assumes, when they found 

 upon it a' separation of the spongke into two great divisions (Amor- 

 phozoaires a squelette corne nnd Amorphozoaires a squelette testace). 



A highly remarkable peculiarity of the sponges here to be described 

 is that they are wanting in any sort of epitheca. All sponges in existing 

 seas adhere to the bottom, either directly, by the lower surface of the 

 body itself, or by means of a pedicle, into which the body narrows it- 

 self at the end. [Only the Tethya Uncurium, Lam., and, as Sars has 

 told me, a small living sponge, growing on the northern coast of Nor- 

 Avay, appears to form an exception among living sponges, being free, 

 and yet it remains to be determined whether they are always and in 

 all stages of their development free.] All well preserved sponges of 

 the Chalk and Jura formations exhibit an epitheca of greater or less 

 size and distinctness. The deviation from this rule in the Silurian 

 sponges is therefore altogether unexpected. Moreover, the preserva- 

 tion is so complete, and the number of individuals so great, that the tact 

 has been determined with certainty. In no one of the numerous exist- 



