Part 54 (1/2)

Ludlow, and Aym. limest.]

[Ill.u.s.tration: Fig. 421. _Leptaena depressa._ Wenlock.]

[Ill.u.s.tration: Fig. 422. _Phacops caudatus_, Brong. Wenlock, Aym.

limest., and L. Ludlow.]

_Leptaena depressa_, Sow., is common in this rock, but also ranges through the Lower Ludlow, Wenlock shale, and Caradoc Sandstone.

[Ill.u.s.tration: Fig. 423. _Catenipora escharoides._]

Among the corals in which this formation is very rich, the _Catenipora escharoides_, Lam. (fig. 423.), or chain coral, may be pointed out as one very easily recognized, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to the bottom of the series.

Another coral, the _Porites pyriformis_, is also met with in profusion; a species common to the Devonian rocks.

_Cystiphyllum Siluriense_ (fig. 425.) is a species peculiar to the Wenlock limestone. This new genus, the name of which is derived from +kystis+, a _bladder_, and +phyllon+, a _leaf_, was inst.i.tuted by Mr. Lonsdale for corals of the Silurian and Devonian groups. It is composed of small bladder-like cells (see fig. 425. _b._).

2. The Wenlock Shale, which exceeds 700 feet in thickness, contains many species of brachiopoda, such as a small variety of the _Lingula Lewisii_ (fig. 412.), and the _Atrypa reticularis_ (fig. 414.) before mentioned, and it will be seen that several other fossils before enumerated range into this shale.

[Ill.u.s.tration: Fig. 424. _Porites pyriformis_, Ehren. Wenlock limest. and shale. Also in Aymestry limestone, and L. Ludlow.

_a._ Vertical section, showing transverse lamellae.]

[Ill.u.s.tration: Fig. 425. Cystiphyllum.

_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 Shrops.h.i.+re. 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 sh.e.l.ls of the genera _Orthoceras_, _Nautilus_, and _Bellerophon_; and among the Brachiopoda the _Pentamerus oblongus_ and _P. laevis_ (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_.

[Ill.u.s.tration: Fig. 426. _Pentamerus laevis_, Sow. Caradoc Sandstone.

Perhaps the young of _Pentamerus oblongus_.

_a, b._ Views of the sh.e.l.l itself, from figures in Murchison's Sil. Syst.

_c._ Cast with portion of sh.e.l.l remaining, and with the hollow of the central septum filled with spar.

_d._ Internal cast of a valve, the s.p.a.ce once occupied by the septum being represented by a hollow in which is seen a cast of the chamber within the septum.]

[Ill.u.s.tration: Fig. 427. Cast of _Orthis grandis_, J. Sow. Horderley; two-thirds of nat. size.]

[Ill.u.s.tration: Fig. 428. _Tentaculites scalaris_, Schlot. Eastnor Park; nat. size, and magnified.]

The most ancient bony remains of fish yet discovered in Great Britain are those obtained from the Wenlock limestones; but coprolites referred to fish occur still lower in the Silurian series in Wales.

[Ill.u.s.tration: Fig. 429. _Ogygia Buchii_, Burmeister. Syn. _Asaphus Buchii_, Brong. 1/4 nat. size. Radnors.h.i.+re.]