224 PEOF. H. A. NICHOLSON AND DK. J. MURIE ON THE 



water-canals are of comparatively large size, and their apertures 

 are conspicuously visible in vertical sections. "We subjoin a brief 

 diagnosis of the type species. 



Pachystroma ANTiQUA, Nich. & Murie. 



Sarcodeme massive, subspberical, of thick concentric laminae, which 

 have the extraordinary thickness of from three quarters of a line to 

 two lines. Interlaminar spaces wanting or irregular. Even when 

 there are no interlaminar spaces, however, the separate laminae are 

 always marked by a distinct interval, which marks a stage of growth, 

 and which if followed laterally is found to expand into irregular inter- 

 spaces. Surface with delicate branching canals arranged in stellate 

 systems. Internal structure of laminae minutely porous and reticulate, 

 especially towards their central parts, with delicate vertical tubes at 

 intervals. 



Form. 8f hoc. — Niagara Limestone, Thorold, Ontario. Coll. Nicholson. 



(y) DiCTYOSTROMA, Nich. 

 (Pal. Ohio, vol. ii. p. 254.) 



Sarcodeme massive, composed of thick solid calcareous con- 

 centric laminse, apparently traversed by horizontal canal^ and 

 separated by wide interlaminar spaces. Upper surfaces of the 

 laminaB sending off strong calcareous processes, which represent 

 the " radial pillars," but seem not to be directly connected with 

 more than the lamina from which they spring. 



Type. — Dictyostroma undulata, Nich. This form resembles 

 Stromatopora proper in general structure, except that the laminae 

 and interlaminar spaces are of comparatively gigantic size. We 

 have had no opportunity of examining its minute structure ; and 

 therefore the group cannot be properly characterized. 



The type species of this group is from the Niagara formation 

 (Upper Silurian) of Kentucky ; and we know of no other form at 

 present. 



Affinities and Systematic Position of the 

 Stromatopoeoids. 

 That the fossils under the so-called Stromatopora include a 

 number of forms at first difficult to collate, has been admitted by 

 most authorities who have investigated the group. As we have 

 shown, certain specimens which, from their general aspect, the 

 localities where obtained, and other reasons, must, for the present 

 at least, be deemed Stromatoporoids, are nevertheless so aberrant, 

 and sometimes so altered in condition and minute structure, as to 

 leave a loop-hole of doubt concerning their nature. Dismissing 



