224 



MINERALS AND GEOLOGY 



(the sponge of commerce), or by spicules or a spicular-skeleton of silica 

 or of carbonate of lime. The spicules are of various forms mostly 

 needle-like, or three-pointed, anchor-like, irregularly -branching, or 

 stellate ; and the shape, and in some cases the arrangement, of these 

 spicules is found to be a more or less constant character, whilst 

 the outer form of the sponge is exceedingly variable. Hence, the 

 modern classification of sponges is based essentially on spicular char- 

 acters ; but these, in fossil examples, are as a rule of somewhat 

 difficult observation. Occasionally, they may be made out if the 

 sponge be dissolved in dilute acid ; but this method of observation is 

 very frequently inapplicable from the entire body of the sponge 

 having become silicified by fossilization. The internal structures can 

 then only be detected by the microscopic examination of thin slices 

 or chippings (ground down with emery powder on a cast-iron plate) 

 under an object glass of tolerably high power. 



Sponges are thus commonly classified as in the annexed table : 

 I. Myxosponyice gelatinous, only. 

 II. Fibrospongice with horny framework, or separate or 



united siliceons spicules, or both. 

 III. Calcisponyice with calcareous spicules. 



Classes II. and III. are sub-divided further into families as (in 

 Class II.) CeraospongiJce (with horny framework, sometimes con- 

 taining simple spicules; ; Monactinellidce (with simple, unbranched 

 siliceons spicules) ; Tetractinellidce (with siliceous four-pronged 

 spicules) ; Lithistidm (with branching, often united, spicules) ; and 

 Hexactinellidce (with six- armed siliceons spicules). The Calcispongice 

 fall into : Ascones (with thin " wall," and regularly-arranged three- 

 rayed and other calcareous spicules) ; Leucones and Pharetrones (with 

 thick wall, traversed by irregularly-branching canals, and with 

 scattered or united spicules) ; and Sycones (with thick wall, traversed 

 by radiating canals, and with regularly-arranged spicules). 



The fossil sponges, or bodies regarded as sponges, hitherto found in 

 the strata of Ontario and Quebec, are very few in number, and all 

 are more or less obscure in character. The principal comprise : 

 Stromatopora ; Archeocyathus (already mentioned under the foram- 

 inifera, but which is probably a calcareous sponge of the order 

 Sycones) \ Eospongia ; and Astylospongia. 



