AFFINITIES OF THE GENUS SIPHONIA. 8.17 



coming slightly irregular or sinuous in outline, and at others bulging 

 out all round into a large protuberance of no very precise form : 

 these are certainly subsequent formations ; and since, in character 

 and position, they resemble some of the tubercular bosses, it seems 

 probable that the latter are subsequent also. Finally, the bosses are 

 hollow within, like the rest of the spicule, and the silica of the 

 interstices radiates away from them in fibrous tufts ; and thus we 

 have repeated a structure and arrangement which I have before 

 described in a very different sponge, viz. Stauronema, one of the 

 Hexactinellidce, and in which certainly they are the result of changes 

 which have taken place during fossilization. The same holds good 

 with the tubercles of the specimen we are describing ; they are not 

 proper to the original spicule, but have been formed as products of 

 its fossilization. In both sponges, in Siphonia and Stauronema, the 

 cast of the spicule has eaten its way outwards from its original 

 position into a number of hemispherical tubercles ; and these have 

 served as centres from which a radiating crystallization of silica has 

 been set up ; in Siplionia, however, the spicular casts have remained 

 empty, but in Staurcnema they have become filled up with a crys- 

 talline carbonate of lime. 



4. Specimens of S. Konigii from the Chalk. — Theseexist in a great 

 variety of mineral states ; but in all the chief fossilizing agent is 

 silica. In examining a common flint nodule which has been split 

 open and found to contain a Choanite, we observe on the fractured 

 surface, most exteriorly, a ring of opaque milk-white silica, exca- 

 vated by a perfect network of empty spaces, on which apparently 

 its white colour, to a great extent, depends ; succeeding this, next in- 

 teriorly, is a zone of dark transparent flint; and next to this, occupy- 

 ing the central area of the broken surface, is a white network, having 

 its canals and interspaces filled with dark transparent flint, like 

 that of the previous or middle zone. The central network is not, 

 however, uniformly white and opaque, but portions of it are con- 

 siderably more transparent than the remainder, the whiter and less 

 white parts differing in appearance just as a piece of ordinary white 

 paper differs from the same paper when impregnated with oil or 

 grease. 



The central network alone represents the original Choanite, the 

 outer and middle zones having accumulated round it during its 

 silicification. The outer zone, however, sometimes contains isolated 

 Lithistid spicules, or, rather, the empty casts of such spicules ; and 

 similar casts sometimes project from the interior of this zone into 

 the clear flint of the succeeding middle zone, wherein they appear as 

 white and solid spicules, the true nature of which is at once revealed, 

 however, by examining their extremities where they are intersected 

 by the plane of fracture. Indeed I may here go so far as to state 

 that whenever one sees a very white and opaque, solid-looking 

 spicule imbedded in clear transparent flint, one may at once expect 

 to find it just the reverse, as regards solidity, of what it seems. 



The middle zone contains numerous transparent spicules of various 

 kinds of sponges, various Foraminifera, and other included bodies. 



