64 PROCEEDINGS OF THE ACADEMY OP [188^. 



The structure of the form called Stylodictyon columnare, found 

 in the Helderberg limestones of Kelley's Island, etc., is somewhat 

 more compact than we find it in the specimens of the Hamilton 

 group named retiforme^ but there occur numerous transition 

 forms from the compact to a more open, porous structure. In 

 the size of the papillose prominences, also, a great variation of 

 forms exists. I think, therefore, we could well spare one specific 

 name and retain the one I had selected for both varieties jointly, 

 Stromatopora Wortheni. 



In the Helderberg limestones of Vernon, Indiana, and at the 

 Falls of Ohio, occurs a form of Stromatopora very similar in 

 structure with the former, but with columellar streaks much 

 smaller than in these, measuring only half a millimetre in 

 diameter, or little over. Its tissue mass is also more bulky, of 

 open reticulated spongious texture plainly' recognizable with a 

 common hand magnifier. I had given in my manuscript descrip- 

 tion and figures of it, under the name Stromatopora granidifera, 

 as the papilli on its surface give it rather a granulated than 

 papillose aspect, on account of their smallness. Horizontal 

 radial ducts are well developed in this form. In it and in the 

 former species these ducts unite in more or less distant centres, 

 which are totally independent of the papillose prominences of 

 the surface, while in many other forms of Stromatopora^ as for 

 instance, in monticulifera and piistulifera Winchell, the apex of 

 each of these monticulose elevations is also the centre of conver- 

 ffence for such channels. 



Under Nos. 5 and 6, generic descriptions of Stromatocerium 

 and of Pachystroma are given, which require no further critical 

 comment, as I have on a previous page sufficiently expressed my 

 views on the correctness of the emended characteristic of Stro- 

 matocerium and of the merits of the distinction of Pachystroma. 



As a seventh generic form of Stromatoporoids a certain con- 

 centricall}' laminated fossil found in the Niagara limestone of 

 Louisville, Ky., has been described by Mr. Nicholson under the 

 name Dictyostroma, which has nothing in common with the 

 Stromatoporoid tribe, excepting an external resemblance, being 

 composed of a succession of laminte separated by large inter- 

 stices. The upper surface of these laminae is covered with 

 spiniform prominences, part of which are long enough to reach 

 the bottom of the superincumbent lamella ; others are shorter. 



