Lign. 136. Univalves from the Mountain Limestone.
| Fig. | 1.— | Euomphalus pentangulatus; Upper surface. |
| 2.— | Polished section of the same species. | |
| 3.— | Natica plicistria. Yorkshire. Mt. L. | |
| 4.— | Pleurotomaria flammigera. (Phil. York.) Mt. L. |
There are two species of this genus (formerly named Cirrus by Mr. Sowerby) which are of frequent occurrence in the White Chalk of England, in the state of casts, and are figured in my Foss. South D. tab. xviii., under the names of Cirrus perspectivus, and Trochus linearis. The Chalk Marl of Sussex yields in some localities (Hamsey, Middleham, Clayton) fine casts of Pleurotomaria, which appear to be distinct from those of the upper cretaceous strata.
EUOMPHALUS.
Euomphalus.[387] [Lign. 136, figs. 1, 2.]—The shells of this extinct genus are deeply umbilicated, discoidal, spiral univalves, having the innermost whorls of the shell divided by imperforated partitions. The internal structure of these shells will serve to prepare the student for those more complicated forms of the testaceous apparatus presented by the Cephalopoda, which will form the subject of the next chapter. There are several recent univalves the animals of which retreat in the progress of growth from the apex of the spire, and the vacated portion is shut off by a shelly plate. In some genera a series of concave septa are thus formed; but in others (as Magilus) the deserted cavity is filled by a compact accretion of calcareous matter, and a solid elongated shell is produced. The Euomphalus, of which there are many species in the Silurian, Devonian, and Carboniferous strata, belongs to the former group. As the animal increased in size, it deserted the smaller and innermost portion of the spire, and a nacreous partition was secreted by the posterior part of the mantle, the interspace remaining hollow; as this process was repeated at different periods, several cells were successively formed. This chambered structure is shown in the specimen [Lign. 136, fig. 2], in which the cells are filled with spar, but the outer cavity is occupied by limestone like that in which the shell was imbedded; a proof that no communication existed between the chamber occupied by the body of the animal, and the space from which it had withdrawn. The calcareous spar, as in the vegetable remains previously described ([p. 71].), has percolated the substance of the fossil, and crystallized in the innermost cells. We shall again have occasion to refer to this interesting fact, when investigating the chambered cells of the Cephalopoda. It may be necessary to remark, that it does not appear that the vacant interspaces in the Euomphalus served the special purpose of the air-chambers of the Nautilus and Ammonite.
[387] So named by Mr. Sowerby, in allusion to the deeply umbilicated character of the disk.
