Fig. 425.
- a. Cystiphyllum Siluriense, Lonsd. Wenlock.
- b. Section of portion, showing cells.
LOWER SILURIAN ROCKS.
The Lower Silurian rocks have been subdivided into two portions.
1. The Caradoc sandstone, which abuts against the trappean chain called the Caradoc Hills, in Shropshire. Its thickness is estimated at 2500 feet, and the larger proportion of its fossils are specifically distinct from those of the Upper Silurian rocks. Among them we find many trilobites and shells of the genera Orthoceras, Nautilus, and Bellerophon; and among the Brachiopoda the Pentamerus oblongus and P. lævis ([fig. 426.]), which are very abundant and peculiar to this bed; also Orthis grandis ([fig. 427.]), and a fossil of well-defined form, Tentaculites annulatus, Schlot. ([fig. 428.]), which Mr. Salter has shown to be referable to the Annelids and to the same tribe as Serpula.
Fig. 426.
