Casts of a large ventricose, globular univalve, called Dolium,[386] have been found in the Chalk Marl, at Clayton, near Hurstpierpoint, in Sussex. This species is distinguished by its transverse tuberculated bands; it is a very rare production of the lower chalk of Sussex (Min. Conch. tab. 326). Turbinated shells related to Trochus, and belonging to several genera, occur in the Cretaceous deposits. As is the case generally with the univalves of this formation, but slight traces of the shells remain; the thin internal nacreous lining is sometimes found adhering to the cast.
[386] This Chalk fossil is not a Dolium: it is probably related to Ringinella incrassata (Geol. Suss. t. xix. fig. 3), one of the Tornatellidæ, a family largely developed in the chalk.
In the Chalk of Touraine, species of the genera Conus ([Lign. 135, fig. 1]) and Solarium ([Lign. 135, fig. 2]) are found with the shells preserved. The specimens figured, [Lign. 135], are selected to familiarize the student with the difference so commonly observable, between the outer surface of the casts, and that of the shells: in both these fossils the shells are marked externally with lines and tubercles; but the casts present only the smooth surface of the interior of the shell in which they were moulded.
Lign. 135. Univalves, from the Chalk of Touraine.—nat.
| Fig. | 1.— | Conus tuberculatus, with part of the shell remaining attached to the cast. |
| 2.— | Solarium ornatum, with the shell. | |
| 2a.— | Specimen of the same species, deprived of the shell. |
In the most ancient fossiliferous formations, the Carboniferous; Devonian, and Silurian, many species and genera of Gasteropoda have been discovered. Professor Phillips enumerates more than ninety in the mountain limestone of Yorkshire (Phil. York.), belonging to the genera Turbo, Pleurotomaria, Natica, Euomphalus, Loxonema, Macrocheilus, Platyceras, and Metoptoma. Thirty-four species from the Silurian rocks are figured and described in Murch. Sil. Syst. p. 706.
The Natica, [Lign. 136, fig. 3], sometimes attains thrice the size represented, and has been found in many localities in England and Ireland.
Pleurotomaria. [Lign. 136, fig. 4.]—This is an extinct genus, distinguished from Trochus by a fissure on the right lip, the position of which is indicated by the band along the back of the whorl in [Lign. 136]; several species occur in the Mountain Limestone; the markings of the original shell are sometimes preserved, as in the example delineated This genus is common in the Oolite; a splendid species, with the shell entire, is found in the Kimmeridge Clay, at Hartwell; limestone casts of the same species are abundant in the Portland stone at Swindon, in Wiltshire.