Pentamerus Knightii, Sow. Aymestry.
- a. view of both valves united.
- b. longitudinal section through both valves, showing the central plate or septum; half nat. size.
3. Aymestry limestone.—The next group is a subcrystalline and argillaceous limestone, which is in some places 50 feet thick, and distinguished around Aymestry by the abundance of Pentamerus Knightii, Sow. ([fig. 411.]), also found in the Lower Ludlow. This genus of brachiopoda has only been found in the Silurian strata. The name was derived from πεντε, pente, five, and μερος, meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five; but neither the structure of this shell, nor the connection of the animal with its several parts, are as yet understood. Messrs. Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia.
Fig. 412.
Lingula Lewisii, J. Sow. Abberley Hills.
Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii ([fig. 412.]); 2d, Terebratula Wilsoni, Sow. ([fig. 413.]), which is also common to the Lower Ludlow and Wenlock limestone; 3d, Atrypa reticularis, Lin. ([fig. 414.]), which has a very wide range, being found in every part of the Silurian system, except the Llandeilo flags.