508 A POPULAR EXPOSITION OF THE 
within, but more commonly it is furnished with a number of radiating 
plates or lamelle. These, in some forms, are but slightly developed, 
or occur only in a rudimentary condition; whilst in others they ex- 
tend far into the cell, and even unite there in a central column. A 
central column or ‘‘axis’’ sometimes, however, exists by itself, and 
may have radiating lamellze of its own projecting towards the cireum- 
ference of the cell ; but this latter modification is not observed in any 
of the Palzeozic types. Whether radiating lamelle are present or not, 
the cell is very generally divided horizontally by a series of transverse 
plates or “diaphragms,” either extending across the entire cell (Fig. 
71, a, which shows three cells thus divided) or occupying the central 
Fig. 71. 
portion of this, whilst the sides are filled with small and more or less 
irregular plates, called “ vesicular tissue,” (Fig. 71, 6). In the genus 
Cystiphyllum, again, the interior of the cell is entirely filled with these 
irregular vesicular plates (Fig. 71, ¢c). Fimally it may be mentioned 
that many corals possess an enveloping wall or sheath. This is 
termed an “‘epitheca.”’ 
The following are the more important or characteristic fossil species 
met with in Canadian rocks : 
1. Stromatocerium rugosum, Fig. 72.— 
In this form, there are no apparent cells, 
but the corallum is made up of numerous 
concentric and wavy lamelle. Lower Silu- 
rian: Trenton group*; more especially 
abundant at the lower part. This fossil 
is also known as stromatopora rugosa, 
and is sometimes classed asa sponge. A Fig. 72. 
closely related species, Stromatopora concentrica, occurs in the Niagara 
group of the Upper Silurian series, and passes in some districts into 
the Devonian rocks. 
* The subordinate divisions of our Silurian and Devonian strata will be fonnd described 
in full in Part V. 
