68 W. WOODLAND. 



crystalline nature of calcite, as a factor in determining form, 

 seems to exhaust itself, in the case of calcareous spicules, in 

 producing the individual crystals, of which these spicules are 

 composed ; certainly there is no reason to suppose that it has 

 any influence on the form of the aggregate (see Maas [15]). 

 With regard to siliceous spicules, hydrated silica (opal), so 

 far as I know, has never been observed to assume a crys- 

 talline form, although some of the siliceous concretions 

 figured by Maas (16) are angular in outline. 



Several authors have contended for the recognition of 

 certain calcareous spicules as crystals on the ground that, 

 like true calcite crystals, they behave optically (with polarised 

 light) as " crystal individuals " (Bidder [1] and others). I 

 have not here the space to discuss such a large subject, but I 

 may point out that the value of this supposed criterion of a 

 crystal is easily shown, in the case of spicules at least, to be 

 naught by the fact that, according to this criterion, the shape 

 of a simple spicule like one of those of Alcyonium is not due 

 to crystallisation (though undoubtedly a spicule individual), 

 whereas the shape of a compound quadriradiate spicule of 

 Calcarea is (though this spicule is composed of four spicule- 

 individuals secondarily joined together to form a system). 



The above arguments, it must be admitted, are quite valid, 

 but they are only entirely valid provided that the term 

 crystal retains the definition I have above supplied. Many 

 facts, however, point to the conclusion that the form of a 

 crystal is as much a function of the medium in which the 

 crystal is dejDOsited as of the properties of the crystal 

 substance itself, and since, as is well knowu, a given 

 crystalline substance will, in the presence of different media, 

 give rise to the most varied forms (these forming, however, a 

 continuous series on account of numerous transitional forms 

 — facets giving place to curved surfaces among other 

 changes), it becomes questionable as to whether we are 

 justified in restrictiug the term crystal to the old meaning. 

 Now the media most potent in these effects on crystalliue and 



