278 PALEONTOLOGY OF ILLINOIS, 



surface of the stem their arrangement is much less regular, 

 while they are also somewhat larger aud more angular than those 

 occupying the space for some distance above the stern. Here 

 their mouths are sub-circular, and separated by sponge tissue 

 less than half their diameter in width. Near the margin of the 

 upper surface the canal apertures again become more angular, 

 and the sponge substance thinner. Numerous smaller canal 

 apertures occupy the spaces between the larger caimls. 



A vertical section shows that the sponge walls are traversed 

 by labyrinth ically intertwined canals. The separating sponge 

 substance, which is generally not more than 0.1 mm. thick, has 

 been replaced by crystalline calcite, and the process has effectu- 

 ally destroyed nearly all the finer details of structure. Where 

 the walls are thinnest, or less compact than usual, obscure 

 traces of the spicules and sponge fibre are preserved. So far as 

 the unsatisfactory conditions will admit of determining their 

 characters, it seems that the fibres contained both three-rayed 

 and uniaxial spicules. 



The above describes the characters of the specimen we have 

 illustrated. The collection contains, besides, two other examples 

 which differ considerably in the size and arrangement of the 

 canals, and may belong to another species of the genus. 

 Although we were tempted to add another species to this pecu- 

 liar genus, we have concluded to await the discovery of other 

 specimens, before deciding them to be distinct. 



Position and locality: Twenty-five feet above the base of the 

 Trenton limestone, near Dixon, Illinois. 



DYSTACTOSPONGIA MINOR. U. & E. 



PL Yin, Fig. 3. 3a, 3b. 



Sponge lobate massive, composed of irregular vertical leaves, 

 which combine to form a tubular mass. The tubes or canals 

 radiate from the base to the upper surface of the specimen, 

 where their walls, which have a thickness varying from 0.1 to 

 0.4 mm., present the appearance of short, beaded lines of cal- 

 cite, forming an incomplete net work, with small and irregular 

 meshes, not exceeding, apparently, 1 mm. in width. 



