276 PALAEONTOLOGY OF ILLINOIS. 



in width from 0.2 to 0.7 mm.; their walls are thin and consist of 

 cemented spicules. The skeleton, being calcined, gives but a 

 faint idea of the individual spicules. 



The internal character of the species, as they appear in thin 

 sections, are well shown in figs. 4a and 4b. In transverse sec- 

 tions, the angles of junction between the canals, are alternately 

 composed of: ( 1 ) clear crystalline calcite, representing the nodes 

 of the spicules, and (2) the matrix which passes through the 

 connecting pores. In fig. 4a the latter are represented by a 

 darker shade. It would, perhaps, have been better to have 

 left them out of the drawing altogether, as the irregular shad- 

 ing by which we intended to imitate the limestone matrix, 

 might be interpreted to represent details, of skeleton structure 

 not intended. 



The distinguishing feature of this species is found in the un- 

 equal size of the radiating canals. The larger ones are gener- 

 ally surrounded by one or two series of the smaller canals, but 

 their arrangement with relation to each other is quite as vari- 

 able as their width. 



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

 Trenton limestone, near Dixon, Illinois. 



STROTOSPONGIA nov. gen. 



Sponges compound, funnel-shaped, composed of thin, intri- 

 cately intertwined vertical leaves. The upper surface with more 

 than one osculum, around which the undulating vertical leaves 

 are arranged in a radiate manner. Cloacal depressions of vari- 

 able depth; the apertures of several vertical tubes occupy the 

 bottom of each. Lower or outer surface with large sub-circular 

 and closely arranged canal apertures, arranged in somewhat 

 regular longitudinal series, while the interstices between them 

 are occupied by smaller canal apertures of variable size and 

 form. 



Vertical sections show that the sponge wall is travered by 

 labyrinthically intertwined canals, having irregularly perforated 

 thin walls. 



