230 



MINERALS AND GEOLOGY 



Before referring to the more typical forms of Canadian occurrence, 

 it will be necessary to explain briefly a few common terms employed 

 in the description of the Hydrocoralla and corals generally. The 

 animal substance of corals consists of soft gelatinous matter contain- 

 ing a digestive sack or stomach (or a number of these, united) with 

 a series of tentacles around the opening or so-called mouth. The 

 gelatinous matter possesses in the great majority of cases the power 

 of secreting amongst its tissues a calcareous or horny framework, 

 technically known as a corallum, the " coral " of popular language. 

 This corallum is either " sclerodermal," or "sclerobasal." A scler- 

 odermal corallum is secreted within the tissues, and reveals more or 

 less the form of the polyp or fleshy part of the animal. It is 

 always composed of a single cell or tube, round, oval, hexagonal 

 or polygonal in shape or of a number of these in close juxtaposi- 

 tion or otherwise united : (1) by two sides only of the cell 

 (Holy sites) ; or (2) by short lateral tubes ( Syringopora) ; or (3) by 

 a mass of more or less porous, or occasionally compact tissue, known 

 as " coenenchyme ". The sclerobasal-corallum, on the other hand, is 

 never cellular. Ts is secreted, as a common support, by the entire 

 fleshy mass of the compound coral, the separate polyps being devel- 

 oped only within this fleshy mass, as seen for example, in the well- 

 known " red coral " of the Mediterranean. 



In the sclerodermal or cellular corallum, the interior of the cell is 

 in some cases quite smooth or open ; in others, the walls are marked 

 ABC D 



FIG. 135. 



by vertical striae ; in others, vertical projections radiate from the walls 

 towards the interior of the cell, and often unite in its centre. These 

 projections, known as " septa " or "radiating septa," present in some 

 groups of forms a very distinct hexamerous character, and in others 

 a tetramerous system of arrangement, but these characters in fossil 

 examples cannot always be made out, and in many genera, the septa 

 are quite variable in number. Sometimes, in place of one of the 

 primary or more strongly pronounced septa, there is a narrow empty 

 space or " septal fossette " as in Zaphrentis, etc., but this in badly 



