AMORPHOZOA. 9 



moudrse," all contain traces of sponge structure, and their 

 origin is in some measure connected with the periodic growth 

 of large crops of sponges. Frequently the crust or outer 

 surface only of the sponge has been silicified, while the centre 

 has decayed, leaving a botryoidal or stalactitic cavity. The 

 cup-shaped sponges are almost always more or less enveloped 

 with flint, which invests the stem and lines the interior, leaving 

 the rim exposed. The sponges of the Yorkshire chalk are of 

 a different character: some are elongated and radiciform, 

 others horizontally expanded, but they contain comparatively 

 little silica ; while those belonging to the genus Manon 

 (fig. 2, 4 ), having prominent " oscula." are superficially silici- 

 fied, and will bear immersion and cleaning with hydrochloric 

 acid. The largest group of chalk sponges, typified by Ventri- 

 culites (fig. 2, 3), have the form of a cup or funnel, slender or 

 expanded, or folded into star-like shape (Guettardia, fig. 2, 2), 

 with processes from the angles to give them firmer attach- 

 ment. Some have a tortuous or labyrinthic outline, and 

 others are branched or compound, like Brachiolites. Curious 

 sections of these may be obtained from specimens enveloped 

 with flint or pyrites. The burrowing sponge, Cliona, is com- 

 monly found in shells of the tertiaries and chalk. The great 

 cretaceous Exogyrce of the United States are frequently mined 

 by them: and flint casts of Belemnites and Inoccrami are 

 often covered by their ramifying cells and fibres. Thin sec- 

 tions of chalk flints, when polished and examined with the 

 microscope, sometimes exhibit minute spherical bodies (Spini- 

 ferites) covered with radiating and multicuspid spines. From 

 their close resemblance to the little fresh-water organism 

 Xanthidium, they long bore that name ; but they are certainly 

 marine bodies, and probably the spores of sponges. 



The generic forms of sponge augment in number and 

 variety from the silurian to the cretaceous beds, where the 

 increase is rapid. But all those, like Siphonia, Sparsifongia, 



