Radiolites Mortoni, Mantell. Houghton, Sussex. White chalk.
Diameter one-seventh nat. size.
Fig. 253. Two individuals deprived of their upper valves, adhering together.
Fig. 254. Same seen from above.
Fig. 255. Transverse section of part of the wall of the shell, magnified to show the structure.
Fig. 256. Vertical section of the same.
On the side where the shell is thinnest, there is one external furrow and corresponding internal ridge, a, b, figs. 255, 256; but they are usually less prominent than in these figures. The upper or opercular valve is wanting.

The general absence of univalve mollusca in the white chalk is very marked. Of bryozoa there is an abundance, such as Eschara and Escharina (Figs. 257, 258). These and other organic bodies, especially sponges, such as Ventriculites (Fig. 238), are dispersed indifferently through the soft chalk and hard flint, and some of the flinty nodules owe their irregular forms to inclosed sponges, such as Fig. 259, a, where the hollows in the exterior are caused by the branches of a sponge (Fig. 259, b), seen on breaking open the flint.

The remains of fishes of the Upper Cretaceous formations consist chiefly of teeth belonging to the shark family. Some of the genera are common to the Tertiary formations, and some are distinct. To the latter belongs the genus Ptychodus ([Fig. 260]), which is allied to the living Port Jackson shark, Cestracion Phillippi, the anterior teeth of which (see [Fig. 261,] a) are sharp and cutting, while the posterior or palatal teeth (b) are flat ([Fig. 260]). But we meet with no bones of land-animals, nor any terrestrial or fluviatile shells, nor any plants, except sea-weeds, and here and there a piece of drift-wood. All the appearances concur in leading us to conclude that the white chalk was the product of an open sea of considerable depth.

The existence of turtles and oviparous saurians, and of a Pterodactyl or winged lizard, found in the white chalk of Maidstone, implies, no doubt, some neighbouring land; but a few small islets in mid-ocean, like Ascension, formerly so much frequented by migratory droves of turtle, might perhaps have afforded the required retreat where these creatures laid their eggs in the sand, or from which the flying species may have been blown out to sea. Of the vegetation of such islands we have scarcely any indication, but it consisted partly of cycadaceous plants; for a fragment of one of these was found by Captain Ibbetson in the Chalk Marl of the Isle of Wight, and is referred by A. Brongniart to Clathraria Lyellii, Mantell, a species common to the antecedent Wealden period. The fossil plants, however, of beds corresponding in age to the white chalk at Aix-la-Chapelle, presently to be described, like the sandy beds of Saxony, before alluded to (p. 293), afford such evidence of land as to prove how vague must be any efforts of ours to restore the geography of that period.

The Pterodactyl of the Kentish chalk, above alluded to, was of gigantic dimensions, measuring 16 feet 6 inches from tip to tip of its outstretched wings. Some of its elongated bones were at first mistaken by able anatomists for those of birds; of which class no osseous remains have as yet been derived from the white chalk, although they have been found (as will be seen on page 299) in the Chloritic sand.