STRUCTURE OF STROMATOPORIDA. 55 
matopore of the great dolomite-beds of the Niagara Limestone. Of 
these many are entirely reduccd to crystalline masses of quartz or 
dolomite, except small portions at the surface, while others have 
become hollow and resemble cavities lined or filled with crystals. 
In the Upper Silurian dolomite of Guelph, in like manner, there are 
specimens which have been converted into a granular dolomite, in 
which, however, the lamin and, in some cases, the canals are more 
_or less apparent. 
The study of Stomatopore in these different conditions throws 
great light on the appearances presented by Hozoon in various states 
of preservation, and forms a guide to the interpretation of these, 
which should be before the mind of every one who desires to form 
correct opinions on the subject. 
Since writing the above, I have seen the remarks of Dr. 
Nicholson on the calcareous nature of Stromatopora, and Zittel’s 
observation of the occasional calcification of the spicules of siliceous 
sponges, as reported in the ‘ Geological Magazine’ for January 1878. 
It had not occurred to me that any one acquainted with Stromato- 
pore would doubt their calcareous nature ; but Nicholson has suf 
ficiently disposed of such doubts by the consideration that the Stro- 
matopore are found silicified only in beds in which corals and shells 
have suffered the same change. Nor had it seemed necessary to 
refer in this connexion to the replacement of siliceous spicules with 
calcite. It is, or should be, well known from the behaviour of sili- 
ceous spicules with alkalies, and when heated, that many of them 
are not purely siliceous, but contain animal matter. ‘This, with the 
more soluble character of their silica, enables them to be changed or 
removed without affecting the siliceous matrix. Hence in the sili- 
ceous sponges from the Niagara Limestone the spicules are some- 
times opaque and granular in appearance, or have disappeared altc- 
gether, or have been replaced with calcite or with iron pyrite. These 
changes are, however, rare, and have no bearing on the calcareous 
nature of Stromatopora. 
ord. Classification of Stromatoporide.—tit is not my purpose to 
enter into any revision of the numerous species of this family, or to 
attempt to summarize the work which has been done with reference 
to the American species by Hall, Winchell, Nicholson, Billings, 
Réminger, and others. It may, however, be useful to state the 
results at which I have arrived with reference to the leading generic 
forms. 
1. Stromatopora (Goldfuss, 1827).—In the original definition by 
Goldfuss the genus is characterized as exhibiting “alternating strata 
of a solid and porous character.” The porous strata in this defini- 
tion are the real lamin, the solid strata are the filled-up chambers ; 
and according as one or the other is preserved, we have in this type 
thin laminze connected by pillars, or thick lamine perforated with 
round holes and separated by vacant spaces. The typical species is 
S. concentrica of Goldfuss, and the genus may be held to include all 
the species with thin or moderately thick lamin connected with 
solid and hollow pillars and perforated with minute pores, or haying 
