MINUTE STRUCTUKE OF STEOMATOPORA AND ITS ALLIES. 223 



Type. — Stromatocerium canadense, Nich. & Murie. 



As previously stated, we have restored Hall's genus Stromato- 

 cerium for the reception of a Stromatoporoid from the Treuton 

 Limestone of Canada (Lower Silurian), which we have reason to 

 think to be allied to Hall's Stromatocerium rugositm, or possibly 

 even identical with it. The species just referred to, however, 

 was very imperfectly defined ; and the genus was not charac- 

 terized in any recognizable manner. There is, however, no doubt 

 that the form figured and described by Hall is a true Stromato- 

 poroid ; and as we know of no other members of this group at this 

 horizon, we have thought it best to restore Hall's genus, rather 

 than to create a new one, though our type is so clearly distinct 

 from all the other Stromatoporoids that we should have been 

 quite justified in giving it a fresh generic designation. As Hall's 

 species, however, cannot be recognized, we shall provisionally 

 describe our form under the name of S. canadense. 



Stromatockrium CANADENSE, Nich. & Murie. 



Spec. char. — Skeleton having the form of large rounded or irregular 

 masses, conspicuously composed of numerous dense concenti'ic laminae, 

 about five of which (with the intervening interlaminar spaces) occupy 

 one line. The interlaminar spaces are open, without radial pillars ; 

 and the mass is traversed by numerous discontinuous vertical canals, 

 from -^-Q to ^V inch, or less, in diameter. Surface-characters unknown. 



Form, ^ Loc. — Trenton Limestone (Lower Silurian), Lake Couchiching 

 and Peterborough, Ontario. Coll. Nicholson. 



(/) Pachysteoma, Nich. & Murie. 



Skeleton massive, composed of numerous concentric calcareous 

 laminae of extraordinary thickness, which are either in contact 

 with one another or are separated by narrow and irregular 

 interspaces. When interlaminar spaces exist, they are open, and 

 there are no " radial pillars." The great concentric laminae are 

 composed of dense and indefinitely, but very minutely, porous and 

 reticulate calcareous tissue, having no regular arrangement, but 

 perforated by numerous very minute, irregular, vertical vermicu- 

 late tubes, without distinct walls. Horizontal radiating groups 

 of water- canals are present. Surface smooth. 



Type. — Pachystroma antiqua, Nich. & Murie. 



The type species of this singular genus is from the Upper 

 Silurian (Niagara Limestone) of Canada ; but we must also refer 

 here the P. {Syringostroma) densa, Nich., of the Devoniaa of 

 Ohio. In this latter form (Pal. Ohio, vol. ii. p. 251) the radiating 



