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: 
670 STRATIGRAPHICAL GEOLOGY. [Boox VI. 
(6), Ambonychia (8), Palearea (5); the gasteropods by Murchisonia, 
Pleurotomaria, Raphistoma, Cyclonema, Euomphalus, and Holopea; the 
pteropods by Tentaculites, Conularia, Theca; the heteropods by 11 
species of Bellerophon and some forms of Maclurea; and the cepha- 
lopods by 47 species belonging to the genera Orthoceras, Cyrtoceras, 
Lituites, &e. 
4, Lower Llandovery Group.—In North Wales the Bala beds about five 
miles §.H. of Bala Lake begin to be covered with grey grits, which 
gradually expand southwards until they attain a thickness of 1000 feet 
in South Wales. These overlying rocks are well displayed near the town 
of Llandovery, where they contain some conglomerate bands, and where 
Mr. Aveline detected an unconformability between them and the Bala 
group below them, so that the subterranean movements had already 
begun, which in Wales marked the close of the Lower Silurian period. 
Elsewhere they seem to graduate downwards conformably into that 
group. ‘They cover a considerable breadth of country in Cardigan and 
Carmarthenshire, owing to the numerous undulations into which they 
have been thrown. Their chief interest lies in the transition which 
they present between the fauna of the Lower and Upper Silurian forma- 
tions. They have yielded in all, according to Mr. Etheridge’s census, 68 
genera and 204 species of fossils, whereof 50 genera and 105 species are 
common to the Bala group below, and 45 genera and 104 species pass up 
into Upper Llandovery rocks above. Some of peculiar fossils are 
Nidulites favus, Meristella crassa, M. angustifrons, and Murchisonia angu- 
lata. Among the forms which come up from the Bala group and dis- 
appear here are the corals Heliolites interstinctus, Petraia subduplicata, and. 
Favosites aspera ; the trilobites Lichas lawatus and Illenus Bowmanni ; the 
brachiopods Orthis Actoniz and O. insularis; the gasteropods Murchisonia 
gyrogoma and Cyclonema crebristria; and the cephalopod Orthoceras 
tenuicinctum. But many of the Lower Silurian forms continue on into 
the Upper Llandovery beds. From the abundance of the peculiar 
brachiopods termed Pentamerus in the Lower, but still more in the Upper 
Llandovery rocks, these strata were formerly grouped together under 
the name of “‘ Pentamerus beds.” ‘Though the same species are found in 
both divisions, Pentamerus oblongus is chiefly characteristic of the upper 
group and comparatively infrequent in the lower, while Stricklandinia 
(Pentamerus) lens abounds in the lower but appears more sparingly in 
the upper. 
The Lower Silurian rocks, typically developed in Wales, extend 
over nearly the whole of Britain, though largely buried under more 
recent formations. They rise into the hilly tracts of Westmoreland and 
Cumberland, where they consist of the following subdivisions in 
descending order : 
(Lower Llandovery not represented.) 
Coniston Limestone and Shale : + = Bala beds. 
Voleanic series (green slates and por- : 
phyries): tuffs and lavas without or-|_ Part of Bala, whole of Llandeilo, 
dinary sedimentary strata except at base, (~ pe trek a part of Arenig 
12,000 ft. . F ormation, 
Skiddaw Slates, 10,000 or 12,000 ft., base Arenig, with perhaps Tremadoc 
not seen . ‘ , : ‘ .J~ | and Lingula Flags. 
Apart from the massive intercalation of volcanic rocks these strata 
present considerable lithological and paleontological differences from the 
