214 PROF. H. A. NICHOLSON AND DR. J. MUEIE ON THE 



cerium rugosum or not, is a point which we have no means of de- 

 termining. 



(h) Another singular form, which we propose to distinguish by 

 the generic title of Pachystroma, and of which we possess many- 

 specimens from the Niagara Limestone of Canada, though clearly 

 Stromatoporoid in its general characters, exhibits features even 

 more singular than the preceding. The fossil exhibits the general 

 form of the Stromatoporoids, consisting of irregular subherai- 

 spheric masses, composed conspicuously of numerous concentric 

 calcareous layers ; but these layers are of comparatively enormous 

 thickness (averaging about a line, but sometimes double this 

 thickness), and though perfectly distinct from one another, they 

 are either in direct contact, or they are only separated from one 

 another by irregular aud narrow intervals. There are therefore 

 no " interlaminar spaces " in the proper sense of the term, and 

 necessarily no "radial pillars." When examined by means of 

 transparent vertical sections, the great concentric laminae are seen 

 to be composed of porous calcareous tissue, principally composed 

 of vertical irregular fibres placed at some distance from one 

 another, and only brought clearlj?^ into view by the use of pola- 

 rized light. The concentric laminae are also perforated by nume^ 

 rous delicate, but irregular and generally remote, vertical tubules ; 

 but their surfaces, as seen in fractured or weathered specimens, 

 cannot be shown by the use of a hand-lens to be minutely porous, 

 though doubtless really so. The surface of weathered specimens 

 also shows radiating, branched, "subdermal " canals, placed round 

 numerous independent centres, precisely as in the so-called Goeno- 

 stromoB. Horizontal sections show simply a loosely reticulate cal- 

 careous network, of an irregular character aud granular structure, 

 with oblique sections of the radiating water-canals. In these, 

 however, as in so many of the thin sections of the calcareous spe- 

 cimens of the Stromatoporoids, the use of polarized light is almost 

 essential to a proper deteimiinatiou of the intimate structure. 



(c) In another group of unquestionable Stromatoporoids, the 

 general structure is extremely like that of the typical Stromato- 

 poroB ; but the horizontal laminae and radial pillars are completely 

 merged in one another, so as to be incapable of separation as di- 

 stinct structures ; though both of them exist in a modified form, 

 and the interlaminar spaces are present as well. One of the 

 forms belonging to this group is very common in the Clinton 

 formation in North America, and often attains very considerable 



