LO¥ER SILURIAN SPECIES. 



SPONGIJS. 



Genus O^EMIDITJM * Goldf. 



CNEMTDiTJiM? Trentonensis, Worthen. 



Cnemidium Trentonensis. Side view, natural size. 



General form discoidal, bight about half as great as the breadth at 

 the summit; summit deeply and broadly concave, tbe base of the con 

 cavity being a little to one side of the center. Furrows both on tlieside 

 and in the concavity numerous at the summit but decreasing rapidly 

 by coalescing below. Hight of a medium-sized specimen 1.50 inch ; 

 greatest breadth at the summit 3 inches in one direction and S.25 inches 

 in the opposite one ; depth of concavity 0.70 inch. 



This fine sponge seems to be more nearly related to the genus Cnemi- 

 dium, established by Goldfuss for certain Jurassic forms, than to Asty- 

 lospongia of Eoemer founded on the Upper Silurian sponges of Ten- 

 nessee. 



Locality and position: Three miles northeast of Dixon, Lee county, 

 Illinois, in the lower division of the Trenton limestone. 



