16 Mr. W. J. Sollaa on Stauronema, a npxo 



a close resemblance to that figured by Bowerbank * from 

 AphrocalUstes {Ijthiteon, Bk.) Beatrix. 



The other spicule occurs in a parallel section of the oscular 

 plato, as a cast, partly liollow, partly filled with iron pyrites ; 

 it is simple, not spined, terminates so obscurely that its ends 

 cann<it be made out, and is imbedded in skeletal fibre in com- 

 pany with the ordinary sexradiate spicules (PI. V. fig. 5). 



Modes of Fossilhation. — The fossilizing material is usually 

 crj-stalline transparent carbonate of lime, or calcite, which 

 fills up the meshes of the network, and .occupies the sexradiate 

 canals of the siliceous fibre ; where it occurs in large quantity, 

 as in the meshes and exciirrent canals, it is traversed by nume- 

 rous cleavage-planes ; and it is usually impure from the 

 presence of a little aluminous matter. The fibre thus enclosed 

 consists of silica, and in a few cases is almost as homogeneous 

 and purely siliceous as when it existed in the living state ; but 

 even in this, its most perfect condition it generally exhibits 

 the marks of decay, not only by the absorption of its interior 

 spicule, but in the presence of numerous hemispherical pits 

 excavated from its exterior to various depths, like those de- 

 scribed by Carter as aflfecting recent spicules t ; from this 

 condition it soon passes through a series of changes, the final 

 result of which is to leave it wholly converted into carbonate 

 of lime. The first step in the process is a granulation of the 

 fibre about the internal canal, which soon extends itself, chiefly 

 by eating its way from within outwards, till at length it 

 reaches the outer boundary of the fibre ; and this, which during 

 the process of change has retained its definite outline, often its 

 transparency as well, yields at last, and the fibre becomes 

 granular all through. The granulation, however, also fre- 

 quently appears at the outside and the inside of the fibre at 

 once, and proceeds from each direction till it meets in the inte- 

 rior. AVhile the granulation is thus progressing, a process of 

 absorption is set up about the interior canal, accompanied by 

 a replacement of the fibre in carbonate of lime ] this change 

 takes place from within outwards, and continues till at length 

 a mere shell of rounded granulations of silica separates the 

 calcite without from that within the fibre ; finally this shell 

 itself disappears, and the exterior and the interior calcite become 

 one. But even then, with this extreme mineralogical change, 

 the original structure is not obliterated : the calcite which fills 

 the internal canal and the interspaces of the meshes is trans- 

 parent and usually colourless, or ■v\'ith a faint yellowish 



• Proc. Zool. Soc. 1869, pi. xxii. fig. 9. 

 t Ann. & Ma^. Nat. Hi.'jt. .spr. 4, vol. xii. p. 4-57. 



