214 PALEONTOLOGY OF ILLINOIS. 



species from Safford's Central Limestone, in Tennessee.* There, in 

 common with the associated fossils, the specimens are siliceous, 

 but the silica has undergone so much alteration that the inter- 

 nal characters are no longer distinguishable. The silica in these 

 specimens has been much more changed than in the similarly 

 preserved sponges from the Astylospongia, beds of western Ten- 

 nessee. Here, the entire sponge consists, usually, of a flinty 

 mass in which the spicules and canals can often be traced out 

 with comparative ease. 



Sponges from which the siliceous skeleton has been dissolved, 

 leaving perfect moulds, is another very common condition of 

 preservation, especially among those of the Niagara group. In 

 these specimens the siliceous spicular network has been entirely 

 removed so that, instead of the solid spicule rays and nodes, 

 we have only empty cavities that conform, however, most per- 

 fectly to the structures which formerly filled them. These moulds 

 generally occur in a matrix of chert or flint which is eminently 

 suitable for retaining the form of the sponge skeleton. In fact, 

 I regard this method of preservation as being more favorable 

 than any other met with in palaeozoic rocks since, by pressing 

 heated gutta-percha into the empty moulds, very perfect casts of 

 the spicules are obtained. By this means I have been enabled 

 to work out the structure of Hindia,, Aulocopium, Astylospongia,, 

 Palteomanon, and other genera. In several instances the arti- 

 ficial casts of Astylospongia, are so perfect, that they show 

 clearly the branching of the spicule rays. Numerous examples 

 are found in the Niagara strata of Tennessee, Kentucky and 

 Indiana, while at Spring Valley, Minnesota, in beds of the same 

 age, it appears that all the sponges are preserved in this condi- 

 tion. In the Burlington limestone of Illinois and Iowa, the 

 Monactinellid sponge described in this report is often preserved 

 in a similar manner. 



The changes produced by fossilization in the structure of the 

 calcareous sponges is generally more serious in its effects than 



* The beds from which these specimens were collected are regarded by Safford as 

 forming part of his Central limestone. Eecent investigations make me doubt that this 

 is their true position. So far as the evidence at hand permits me to judge I should say 

 that they constitute a distinct horizon between his Glade and Carters Creek limestones 



