254 PALAEONTOLOGY OF OHIO. 



Gentjs DICTYOSTROMA, NicholsoD. 



Sponge-mass forming a polymorphous or hemispherical mass of con- 

 siderable size, composed of concentrically arranged undulating layers, 

 which surround an imaginary center, but which are not composed of 

 finer laminae. These layers apparently have their substance traversed 

 by horizontal canals, and each sends up from its upper surface a series of 

 prominent, pointed, and apparently hollow processes or papillae, which, 

 without being amalgamated with it, support the layer next above, like 

 so many pillars. The outermost layer of all, forming the exterior of the 

 whole mass, is destitute of these processes, smooth, and apparently solid, 

 except for the presence of irregularly disposed rounded or circular per- 

 forations (oscula), which penetrate the mass more or less vertically, and 

 have a comparatively large size. 



This genus is nearly allied to Stromatopora, especially to such forms as 

 S. tuberculafa, Nich.; but it exhibits differences which seem to be of 

 generic value. The sarcodeme is composed of concentrically arranged 

 layers, but these are of comparatively enormous thickness, and are not 

 composed of subordinate layers. The concentric laminae, when viewed 

 in section, further exhibit oval or rounded apertures, which appear to be 

 the sections of horizontal tubes. In Stromatopora, again, the concentric 

 laminae are traversed by a fine and c\ -"ly arranged system of vertical 

 and apparently solid dissepiments, vv.i en often appear to pass continu- 

 ously through several layers, and are amalgamated with the laminse 

 which they separate. In Dictyostroma, on the other hand, the different 

 layers of which the mass is composed give off from their upper surfaces 

 a series of strong, apparently tubular, more or less vertical pillars or 

 processes, upon the apices of which the next layer above seems to repose, 

 without the occurrence of any direct union between the two, so far as 

 can be determined. Distinct oscula are present in Dictyostroma, as in 

 most of the species of Stromatopora, but I have detected no evidence of 

 " pores." 



The following is the only species of the genus which has come under 

 my notice : 



Dictyostroma undulata, Mcholson. 



Plate 24, figs. 6, 6c. 



Sarcodeme composed of concentrically arranged layers, which have a 

 thickness of nearly one-third of a line, and are separated by intervals of 

 nearly the same width, so that about four layers are present in a vertical 



