8 PALAEONTOLOGY 



surface only of the sponge has been silicitied, 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- 

 tied, 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 Bdcmnites 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 {Sjpinir 

 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 arc certainly 

 marine bodies, and probably the spores of sponges. 



Class II.— RHIZOPODA. 



The organisms of this class are small and for the most 

 pari of microscopic minuteness, of a simple gelatinous 

 structure, commonly protected by a shell. The most simple 



