rock of North America, is in many places divided into laminæ by the remains of innumerable shells of the genus Lingula. They are in such profusion as to form black seams like mica, and are accompanied with another small placunoid shell, which is also associated with a small species of Lingula in the lowest beds of the Llandeilo series of Wales. Here, then, in the most ancient term of organic life is a shell belonging to a genus not extinct, and very like a species still living.*

A species of Ungulites or Obolus (a small orbicular horny shell) occurs in the inferior limits of the fossiliferous deposits in Russia,† occupying the same geological position as the Lingule in the Lower Silurian beds of Wales and North America above cited.

Ambonychia, Avicula, Pterinea, Arca, Cleidophorus, Modiolopsis, Nucula, and Orthonota are most of the principal bivalves. The Pteropoda, such as Conularia‡ and Theca,§ were plentiful; and of large size, compared with the existing species. Of Gasteropoda there were numerous forms: Euomphalus, Holopella, Murchisonia, Pleurotomaria, and some shells resembling the recent Trochus and Turbo, were predominant. Acroculia (like a Capulus) was not unfrequent in some localities, and attained a large size. Chiton-like gasteropods (Helminthochiton) also existed. The Bellerophon, a genus which, like many of its associates, lived on into the Carboniferous seas, had many species in the Silurian. The Cephalopoda, however, were the master-forms** of * Lyell's "Travels, N. America,” vol. ii. p. 157 + "Geology of Russia," vol. ii. p. 292.

Specimens more than a foot long are preserved in M. Barrande's invaluable collection of Bohemian fossils.

§ In the Lower Silurian, five inches is a not unusual length for the Theca; they are smaller in the Upper Silurian: the living species are of microscopic size.-Barrande.

Five inches long, in Bohemia.—Barrande,

¶ Medals, p. 447.

** Nearly 300 species, some of them of large forms, occur in Bohemia. -Barrande.

these primeval seas. In Britain alone nearly sixty species of Silurian Orthocerata are known: in the Bohemian basin M. Barrande has worked out very many more; besides many other genera, some almost as rich in species. M. Barrande has met with at least two specimens of Orthoceras in which remains of the soft parts are still present, in the condition of a soft, waxy, yellowish-brown, adipocire-like substance.* Some specimens of Orthoceras attained a large size, being three feet in length, and having seventy septa.

Goniatites do not occur in the British Silurian strata; but are present (fourteen species) in those of Bohemia, where other fossils of a "Devonian" character are mingled with those of the "Upper Silurian." M. Barrande has found indications of the original colours of the shells retained on eight or ten specimens of Silurian cephalopods of Bohemia. Lituites, Phragmoceras, Cyrtoceras, and Ascoceras are the remaining genera of the British Silurian cephalopods.†

21. SILURIAN CRUSTACEA.-The higher or Malacostracous members of the Crustacean order do not appear in the lower paleozoic rocks. The Entomostraca, however, abound. The Pterygotus, allied to the Limulus, but more nearly resembling one of the Lobster family in outward form, occurs in the Lower Ludlow,‡ with Limuloides, and again in the uppermost of the Ludlow beds, as well as in the Old Red and Lower Carboniferous, as already mentioned. Hymenocaris is a bivalvular phyllopod, somewhat resembling a Nebalia, and is one of the oldest of known organic remains. Ceratiocaris, another bivalved crustacean, with protruded

* For this and much other interesting information respecting the Silurian fossils of Bohemia, the Editor is indebted to the kindness of M. Barrande.

+ The Editor takes this opportunity of acknowledging, with much pleasure, the great assistance he has derived throughout this work from Prof. Phillips's Synoptical Tables of Fossil Genera ("Manual of Geology," 1855), carefully constructed from Morris's "Catal. Brit. Foss.," 1854.

Salter, Annals N. H. 2nd ser. vol. xx. p. 321

tail, belongs to the Upper Silurian. Small bivalved entomostraca, such as Beyrichia and Leperditia, occur both in Lower and Upper Silurian rocks, and often in great numbers. Track-marks, referred to Limuloid crustaceans, and termed Protichnites,* have been founded in the Potsdam

LIGN. 192.-TRILOBITE IN SILURIAN LIMESTONE FROM DUDLEY.
(Calymene + Blumenbachii.

sandstone of Canada, ‡ in the Clinton rocks of the United States, and in the Lower Silurian of Scotland: || others, referred by Mr. Salter to the Hymenocaris, have been found by him in the Lingula-flags of North Wales.¶

*First-tracks.

+ Calymene, signifying concealed: in allusion either to the obscurity of its zoological relations when first studied, or to the non-discovery of legs and antennæ.

Logan and Owen, Quart. Journ. Geol. Soc. vol. viii. pp. 199 and 214. § Desor and Owen, loc. cit. p. 213

|| Harkness, Geol. Soc. Journ. vol. xii. p. 243.

Quart. Geol. Journ. vol. x. p. 208.

Trilobites.*-But the most extraordinary feature in the Cambro-Silurian fauna is the abundance and variety of the

LIGN. 195.-HOMALONOTUS DELPHINOCEPHALUS, FROM THE WENLOCK ROCKS. (Reduced from Pl. VII. bis, fig. 1, Sil. Syst.)

Trilobites, a peculiar family of Crustaceans, of which there are no living representatives, and which is restricted to the * Trilobites, signifying "three-lobed," from the general form of the carapace or shell.

palæozoic formations, and almost exclusively to the most ancient fossiliferous deposits; for, while the Silurian rocks teem with the relics of hundreds of species,* but few, comparatively, occur in the Devonian and Carboniferous.

These remarkable crustaceans had the body protected by a strong dorsal case or shell composed of numerous annular segments, and generally divided into three lobes by two longitudinal furrows or depressions. The head and the tail are each covered by a single piece. The eyes of most of the genera are very large and reticulated, consisting of numerous distinct facets or lenses, as in other crustaceans, and are implanted on the cephalic buckler. No traces of pats, feet, or swimmers have been detected, and it is therefore supposed that these appendages were composed of a soft and perishable substance.†

One of the most common species of Trilobite is the Calymene Blumenbachii, commonly known as the “Dudley fossil Insect" or "Locust," and which has long attracted the attention of collectors; this crustacean is found either attached by the under surface to the rock, as in Lign. 192, or coiled up like an Oniscus, or wood-louse.§ Some kinds, as the Calymene, could coil themselves into a ball like the wood-lice; while others had the central segments alone moveable. The Trilobites vary exceedingly in form and magnitude; some not exceeding a few lines, while others are eighteen or twenty inches in length.

* In Bohemia alone 252 species have been recognised by M. Barrande. See his richly illustrated work on the Silurian Basin of Bohemia.

+ See Medals of Creation, pp. 532-542, for an account of the Trilobites, and for references to monographs and descriptions. The notices of the Trilobites in Sir R. Murchison's "Siluria" must, however, be carefully consulted, as comprising the latest and most complete account of the distribution of this genus.

This Trilobite was figured and described by Lhwyd in 1698. § Medals of Creation, p. 533 (Lign. 175, fig. 4).