of iron.”—­(Sir Wyville Thomson.) Many of the Chalk sponges, originally calcareous, have been converted into flint subsequently; but the Ventriculites are really composed of this substance, and are therefore genuine “Siliceous Sponges,” like the existing Venus’s Flower-Basket (Euplectella).  Like the latter, the skeleton was doubtless originally composed, in the young state, of disconnected six-rayed spicules, which ultimately become fixed together to constitute a continuous frame-work.  The sea-water, as in the recent forms, must have been admitted to the interior of the Sponge by numerous apertures on its exterior, subsequently escaping by a single large opening at its summit.

[Illustration:  Fig. 187—­Kotalia Boueana.]

[Illustration:  Fig. 188.—­Siphonia ficus.  Upper Greensand.  Europe.]

[Illustration:  Fig. 189.—­Ventriculites simplex.  White Chalk.  Britain.]

Amongst the Coelenterates, the “Hydroid Zoophytes” are represented by a species of the encrusting genus Hydractinia, the horny polypary of which is so commonly found at the present day adhering to the exterior of shells.  The occurrence of this genus is of interest, because it is the first known instance in the entire geological series of the occurrence of an unquestionable Hydroid of a modern type, though many of the existing forms of these animals possess structures which are perfectly fitted for preservation in the fossil condition.  The corals of the Cretaceous series are not very numerous, and for the most part are referable to types such as Trochocyathus, Stephanophyllia, Parasmilia, Synhelia (fig. 190), &c., which belong to the same great group of corals as the majority of existing forms.  We have also a few “Tabulate Corals” (Polytremacis), hardly, if at all, generically separable from very ancient forms (Heliolites); and the Lower Greensand has yielded the remains of the little Holocystis elegans, long believed to be the last of the great Palaeozoic group of the Rugosa.

[Illustration:  Fig. 190.—­Synhelia Sharpeana.  Chalk, England.]

As regards the Echinoderms, the group of the Crinoids now exhibits a marked decrease in the number and variety of its types.  The “stalked” forms are represented by Pentacrinus and Bourgueticrinus, and the free forms by Feather-stars like our existing Comatuloe; whilst a link between the stalked and free groups is constituted by the curious “Tortoise Encrinite (Marsupites).  By far the most abundant Cretaceous Echinoderms, however, are Sea-urchins (Echinoids); though several Star-fishes are known as well.  The remains of Sea-urchins are so abundant in various parts of the Cretaceous series, especially in the White Chalk, and are often so beautifully preserved, that they constitute one of the most marked features of the fauna of the period.  From the many genera of Sea-urchins which occur in strata of this age, it is difficult to select characteristic types; but the genera Galerites (fig. 191), Discoidea (fig. 192), Micraster, Ananchytes, Diadema, Salenia, and Cidaris, may be mentioned as being all important Cretaceous groups.