The sponges of the chalk belong to several distinct families. Choanites resembles the Siphonia, but is sessile, and exhibits in section, or in weathered specimens, a spiral tube winding round the central cavity. It is the commonest sponge in the Brighton brooch-pebbles. Others are irregularly cup-shaped and calcareous ; and many of the Wiltshire flints have a nucleus of branching sponge (S. clavellata). The chalk flints, arranged in regular layers, or built up in columns of " Para moudrae," 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
odites (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 Beleinnites and Inoceranti 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.