In the Tremadoc slates are found Bellerophon, Orthoceras, and Cyrtoceras, all specifically distinct from Lower Silurian fossils of the same genera: the Pteropods Theca (Fig. 568) and Conularia range throughout these slates; there are no Graptolites. The Lingula (Lingulella) Davisii ranges from the top to the bottom of the formation, and links it with the zone next to be described. The Tremadoc slates are very local, and seem to be confined to a small part of North Wales; and Professor Ramsay supposes them to lie unconformably on the Lingula flags, and that a long interval of time elapsed between these formations. Cephalopoda have not yet been found lower than this group, but it will be observed that they occur here associated with genera of Trilobites considered by Barrande as characteristically Primordial, some of which belong to all the divisions of the British Cambrian about to be mentioned. This renders the absence of cephalopoda of less importance as bearing on the theory of development.

Lingula Flags.—Next below the Tremadoc slates in North Wales lie micaceous flagstones and slates, in which, in 1846, Mr. E. Davis discovered the Lingula (Lingulella), Fig. 570, named after him, and from which was derived the name of Lingula flags. These beds, which are palæontologically the equivalents of Barrande’s primordial zone, are represented by more than 5000 feet of strata, and have been studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in North Wales, and at St. David’s in South Wales. They have yielded about forty species of fossils, of which six only are common to the overlying Tremadoc rocks, but the two formations are closely allied by having several characteristic “primordial” genera in common. Dikelocephalus, Olenus (Fig. 571), and Conocoryphe are prominent forms, as is also Hymenocaris (Fig. 569), a genus of phyllopod crustacean entirely confined to the Lingula Flags. According to Mr. Belt, who has devoted much attention to these beds, there are already palæontological data for subdividing the Lingula Flags into three sections.[[3]]

In Merionethshire, according to Professor Ramsay, the Lingula Flags attain their greatest development; in Carnarvonshire they thin out so as to have lost two-thirds of their thickness in eleven miles, while in Anglesea and on the Menai Straits both they and the Tremadoc beds are entirely absent, and the Lower Silurian rests directly on Lower Cambrian strata.

LOWER CAMBRIAN.

Menevian Beds.—Immediately beneath the Lingula Flags there occurs a series of dark grey and black flags and slates alternating at the upper part with some beds of sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds were formerly classed, on purely lithological grounds, as the base of the Lingula Flags, but Messrs. Hicks and Salter, to whose exertions we owe almost all our knowledge of the fossils, have pointed out[[4]] that the most characteristic genera found in them are quite unknown in the Lingula Flags, while they possess many of the strictly Lower Cambrian genera, such as Microdiscus and Paradoxides. They therefore proposed to place them, and it seems to me with good reason, at the top of the Lower Cambrian under the term “Menevian,” Menevia being the classical name of St. David’s. The beds are well exhibited in the neighbourhood of St. David’s in South Wales, and near Dolgelly and Maentwrog in North Wales. They are the equivalents of the lowest part of Barrande’s Primordial Zone (Étage C). More than forty species have been found in them, and the group is altogether very rich in fossils for so early a period.

The trilobites are of large size; Paradoxides Davidis (see Fig. 572), the largest trilobite known in England, 22 inches or nearly two feet long, is peculiar to the Menevian Beds. By referring to the Bohemian trilobite of the same genus ([Fig. 576]), the reader will at once see how these fossils (though of such different dimensions) resemble each other in Bohemia and Wales, and other closely allied species from the two regions might be added, besides some which are common to both countries. The Swedish fauna, presently to be mentioned, will be found to be still more nearly connected with the Welsh Menevian. In all these countries there is an equally marked difference between the Cambrian fossils and those of the Upper and Lower Silurian rocks. The trilobite with the largest number of rings, Erinnys venulosa, occurs here in conjunction with Agnostus and Microdiscus, the genera with the smallest number. Blind trilobites are also found as well as those which have the largest eyes, such as Microdiscus on the one hand, and Anoplenus on the other.

LONGMYND GROUP.