[Illustration: Fig. 206. _Plagiostoma Hoperi_, Sow. Syn. _Lima Hoperi_. White chalk and upper greensand.] [Illustration: Fig. 207. _Plagiostoma spinosum_, Sow. Syn. _Spondylus spinosus_. Upper white chalk.] Among the rest, no form marks the cretaceous era in Europe, America, and India, in a more striking manner than the extinct genus _Inoceramus_ (_Catillus_ of Lamk.), the shells of which are distinguished by a fibrous texture, and are often met with in fragments, having, probably, been extremely friable. [Illustration: Fig. 208. _Inoceramus Lamarckii._ Syn. _Catillus Lamarckii_. White Chalk (Dixon's Geol. Sussex, Tab. 28. fig. 29.)] [Illustration: Fig. 209. _Eschara disticha._ _a._ Natural size. _b._ Portion magnified. White chalk.] [2 Illustrations: Fig. 210. Fig. 211. A branching sponge in a flint, from the white chalk. From the collection of Mr. Bowerbank.] With these mollusca are many corals (figs. 209, 210, 211.) and sea urchins (fig. 212.), which are alike marine, and, for the most part, indicative of a deep sea. They are dispersed indifferently through the soft chalk, and hard flint, and some of the flinty nodules owe their irregular forms to inclosed zoophytes, as in the specimen represented in fig. 211., where the hollows in the exterior are caused by the branches of a sponge seen on breaking open the flint, fig. 210. [Illustration: Fig. 212. _Ananchytes ovata_. White chalk, upper and lower. _a_. Side view. _b_. Bottom of the shell on which both the oral and anal apertures are placed; the anal being more round, and at the smaller end.] Of the singular family called _Rudistes_, by Lamarck, hereafter to be mentioned, as extremely characteristic of the chalk of Southern Europe, a single representative only (fig. 213.) has been discovered in the white chalk of England. [4 Illustrations: _Hippurites Mortoni_, Mantell. Houghton, Sussex. White chalk. Diameter one seventh of nat. size. Fig. 213. Two individuals deprived of their opercula, adhering together. Fig. 214. Same seen from above. Fig. 215. Transverse section of part of the wall of the shell, magnified to show the structure. Fig. 216. 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. 213, 214.; but they are usually less prominent than in these figures. This species has been referred to _Hippurites_, but does not, I believe, fully agree in character with that genus. I have never seen the opercular piece, or _valve_, as it is called by those conchologists who regard the _Rudistes_ as bivalve mollusca. The specimen above figured was discovered by the late Mr. Dixon.]