The Llandovery rocks were so named by Sir R. Murchison from the locality in South Wales where they are most fully developed, and where their physical relations to the formations above and below them are clearly exposed. Those rocks constitute an intermediate place in the Silurian table, connecting, by some of their contained organic remains, the fauna of the upper and lower Silurian groups. The upper Llandovery rock contains many shells common to the Wenlock beds, and is considered to form the natural base of those deposits; on the other hand, the lower Llandovery is related by its fossils to the Caradoc beds below, and both members of this formation are united by having certain fossils in common and peculiar to them, but are chiefly characterized by the abundance of some species of Petraia, Atrypa, and especially Pentamerus. This group consists of soft, argillaceous sandstones and building stones (Goleugoed), shales, some calcareous bands, hard, coarse grits, and quartzose, pebbly conglomerates. Thickness 2,000 to 3,000 feet.

Localities.—Both members of this group, and their relations to the overlying and subjacent rocks, are well exhibited at Noeth Grüg, near Llandovery, South Wales. In Radnorshire, Shropshire, and Hereford, the upper member, or Pentamerus oblongus zone, is alone present, and is seen in some places to be unconformable to the Longmynd and Caradoc rocks. The conglomerates of the west flank of the Malverns, and the sandstones of May Hill and Huntly, Gloucestershire, are of this age.