AFFINITIES OF THE GENUS SIPHONIA. 813 



authors, may be explained 1)}- reference to this kind of growth ; for 

 in cases where the last coating had grown so thick that a fresh set 

 of excurrent canals was on the point of appearing in a new layer, 

 but had not actually done so, it would require a considerable 

 amount of attrition to reveal the outermost series of canals ; and the 

 chances are that the exterior of the specimen would be smooth, i. e. 

 not grooved by superficial canals. If, on the other hand, a new set 

 of longitudinal canals had just been produced, then the mere dis- 

 solution of the dermal covering of the sponge would leave them 

 exposed as well-marked grooves proceeding from the rim of the 

 cloaca for a variable distance down the sides of the body, as already 

 described (p. 805). 



4. States of Mineralization - . 



1. Phosjohatic Specimens from the Gaidt. — The infilling material of 

 these fossils is a brown substance composed of calcic carbonate and 

 phosphate, clear and transparent in thin sections, especially in those 

 parts where it fills the intermeshes of the skeleton, since in the 

 canals of the sponge it is rendered more or less opaque by included 

 earthy material, consisting partly of Gault clay and partly of various 

 small foreign bodies, such as glauconitic granules, minute Eora- 

 minifera, as well as contorted fibres of the same kind as those 

 figured in the ' Geological Magazine,' Decade ii. vol. hi. pi. xiv. 

 figs. 8, 9, 10, as resembling contort spicules, and which I hope 

 to show in a subsequent paper to be algoid growths, like that of 

 Saprolegnia, that have infested the sponge subsequent to its death 

 and during its decay. 



The skeletal network in these specimens is generally transparent 

 and colourless ; it dissolves with effervescence in hydrochloric acid, 

 and behaves with a power of GO diam., under polarized light, as pure 

 calcite, of which mineral we may therefore conclude it is mainly 

 composed. When examined, however, with polarized light under a 

 higher power, say 140 diam., small portions of some of the spicules, 

 but not of all, give here and there the colours of quartz ; and when 

 such spicules are treated with hydrochloric acid an exceedingly small 

 insoluble residue of a transparent mineral remains behind, which, 

 when again examined with NicoFs prisms, turns out to be pure quartz. 

 From this it is therefore evident that in some spicules a part, though 

 a very insignificant part, possesses a siliceous composition, while on 

 the other hand by far the larger portion of the skeletal network 

 consists wholly of calcite. Now the skeleton of the Lithistidse is 

 siliceous ; i. e. it consists of organic matter and silica in intimate 

 combination; and since the skeleton of Svplionia is, as we have 

 already shown on morphological grounds, that of a Lithistid sponge, 

 it also must originally have been made up of organic silica, while 

 its present calcareous state can only be due to a subsequent mineral 

 replacement. In spite, however, of the fact that its original siliceous 

 substance has been almost altogether exchanged for a crystalline 

 calcareous one, and in some cases entirely so, it yet exhibits an 



