STRUCTURE OF STROMATOPORID A. 51 
few other fossils. They are also abundant in the Dolomite of the 
Guelph Limestone ; and it is perhaps not accidental that both here 
and in the Laurentian, fossils of this structure are associated with 
magnesian rocks. They occur also in the Lower Silurian, though 
less abundantly ; and the oldest specimen I have seen is in the Pots- 
dam Sandstone ; and this, its structure not being preserved, may have 
belonged to Hozoon rather than to Stromatopora. The Lower Silu- 
rian species have usually very thin and continuous walls. In the 
great Niagara Limestone, as seen at Niagara Falls, the masses of 
Stromatopora occur precisely as Hozoon occurs in the Laurentian 
limestones, and are mineralized with quartz and dolomite, and often 
almost entirely converted into crystalline masses, though occasion. _ 
ally showing their structure in great perfection. 
Certain beds of the Niagara formation, near Hamilton, contain 
not only Stromatopore, but multitudes of sponges ; and through the 
kindness of Lieutenant-Colonel Grant, of that place, I have been 
enabled to examine a number of specimens of these, and to compare 
them with Stromatopore. These sponges are all siliceous and spicu- 
late, and belong chiefly to two or three species of Astylospongia of 
Romer, and to Aulocopina of Billings, of which his A. Grantw is 
the type. The species of Astylospongia present a most regular and 
beautiful hexactinellid structure, as perfect as that in the sponges 
of the Cretaceous, showing even the hollow nodes, which have been 
supposed to be absent in the Paleozoic Hexactinellidse. Auloco- 
pina has a different structure, presenting series of hexagonal tubes 
built up with interlaced spicules, and giving off bundles of spicules 
in a radiating manner. These sponges are thus entirely distinct, 
both in material and structure, from the contemporary Stromato- 
pore, and there is no link of connexion whatever. 
The species included in the genera Caunopora of Phillips and 
Cenostroma of Winchell, and in part in Syringostroma of Nicholson, 
and which may be represented by the Stromatopora polymorpha of 
Goldfuss, have the horizontal canals largely developed in laminew 
thickened by supplemental deposit, and traversed by an infinity of 
minute canaliculi or ramifications of the canals opening at their 
surfaces. The horizontal canals radiate from central points where 
they are connected with vertical tubes or groups of tubes penetrating 
the whole thickness of the mass (Pl. LV. fig. 9, and Pl. V. fig. 10). 
The whole organism thus becomes divided into a series of vertical 
systems, which often very much obscure the concentric lamination, 
and in different states of preservation give very perplexing appear- 
ances. They may all be explained by bearing in mind that the hori- 
zontal canals, like those of Stromatopora proper, pass in the substance 
of the laminze, now much thickened, and that at the centres of the 
systems they descend through the chambers by vertical tubes or groups 
of tubes which correspond to the hollow pillars of Stromatopora. 
A great number of specimens of Cauwnopora, Coenostroma, and 
allied forms, both European and American, have passed through my 
hands; but I was unable to decide, except inferentially, as to their 
minute structure, till I was so fortunate as to obtain, through the 
r2 
