SPONGES. 279 



in vertical sections the canal walls decrease slightly in thick- 

 ness from below upwards. They are pierced at frequent but 

 unequal intervals by large irregular openings. The canal cavi- 

 ties are traversed by fine branching or simple fibres of calcite, 

 which we suppose to represent spicules. In transverse sections 

 the sponge substance is represented by small irregular masses 

 of crystalline calcite. 



Of this species Ave have seen only the small lobate example, 

 illustrated on plate VIII.* The figure gives an oblique view of 

 the specimen and shows, on the left side, vertical canals running 

 from the base to the rounded upper surface. The vermiculate 

 lines on the half to the right, represent their incomplete walls. 

 On account of the great irregularity of the internal structure, it 

 is difficult to draw up satisfactorily descriptions of the species of 

 this genus. The same difficulty is experienced in describing the 

 internal construction of Strotospongia. The structure of these 

 sponges might, perhaps, have been better defined by saying, 

 that the sponges are composed of irregular reticulate fibres so 

 arranged that they form labyrinthically intertwined canals, or, 

 more or less irregular, vertical tubes. 



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

 Trenton limestone near Dixon, 111. 



DYSTACTOSPONGIA RUDIS U. & E. 



PL Vm, Figs. 4-4a. 



Of this species also, the collection contains but one example. 

 This is massive, of oval shape, three and one-half inches long, 

 two and one-half inches wide, and one and five-eighths inches 

 high. It is composed of reticulate sponge fibre arranged in 

 such a manner that it forms very thin, cribrose leaves, .1 mm. 

 or more in thickness, which combine with each other at unequal 

 intervals, so as to produce an irregularly tubular mass. The 

 canals so formed vary in width from 1.5 mm. to 4 mm., and 

 traverse the mass in an irregular radiate manner from the 

 base. 



Since this was written Mr. E. O. Ulrich visited Dixon and succeeded in collecting 

 several other specimens of this and the following species. These agree in all respects 

 with the ones described. 



