Amongst the Cœlenterates, 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
Fig. 190.—Synhelia Sharpeana. Chalk, England. 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 Palæozoic group of the Rugosa.
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 Comatulœ; 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.
Coming to the Annulose Animals of the Cretaceous period,
Fig. 191.—Galerites albogalerus, viewed from below, from the side, and from above. White Chalk. there is little special to remark. The Crustaceans belong for the most part to the highly-organised groups of the Lobsters
Fig. 192.—Discoidea cylindrica; under, side, and upper aspect. Upper Greensand. and the Crabs (the Macrurous and Brachyurous Decapods); but there are also numerous little Ostracodes, especially in the fresh-water strata of the Wealden. It should further be noted that there occurs here a great development of the singular Crustaceous family of the Barnacles (Lepadidœ), whilst the allied family of the equally singular Acorn-shells (Balanidœ) is feebly represented as well.
Passing on to the Mollusca, the class of the Sea-mats and Sea-mosses (Polyzoa) is immensely developed in the Cretaceous period, nearly two hundred species being known to occur in the Chalk. Most of the Cretaceous forms belong to the family of the Escharidœ, the genera Eschara and Escharina (fig. 193) being particularly well represented. Most of the Cretaceous Polyzoans are of small size, but some attain considerable dimensions, and many simulate Corals in their general form and appearance.