Crustacea of the Upper Arenig group, the singular genus Trinu- deus also appearing in the Middle Arenig for the first time. Nume rous genera and peculiar species of Rhabdophora occur at Llanvirn and Ramsey Island. The fauna is essentially Silurian, for with the exception of a few Brachiopoda and three genera of Trilobita, no other Cambrian genera pass up from the Tremadoc rocks. These three are Asaphus, Agnostus, and Ogygia, species of which occur in the Upper Tremadoc. The occurrence here of Placoparia is impor tant as bearing both upon distribution and correlation. The Gas teropoda, through Pleurotomaria llamrirnensis and Ophileta or Rap- histoma, first appear in Britain in the Upper A renigs of St. David’s. We have no record of any Pelecypoda through the whole of the Arenig group in South Wales ; but 4 species (Palcearca amygdalus, Redonia anglica, Ribieria complanata, and Ctenodonta), are known from the Stiperstones area in Shropshire, and 2 (Paloearca socialis and Ctenodonta) from Ty-obry in North Wales. The value of this upper division of the Arenig group in South Wales is enhanced by its re semblance to Barrande's Etage D 1 (Bohemia), and its close relation to the fauna found in the Arenig slates in North-West France. The recognised presence of this stage in Britain enables us to corre late our beds with those on the Continent, and thus establish a general succession for the Western European and British rocks of this age. Sequence of the Arenig Rocks.—The sequence of the Arenig group at St David’s and North Wales is perfect, there being no evidence of any important physical changes having taken place through or during the period or epoch in which the whole series was deposited. The three divisions of lower, middle, and upper are characterised by a distinctive fauna, few species being common to all the three groups or divisions, but a general facies pervades the whole. Research at Portmadoc, the Ffestiniog area, and west of the Arenigs exhibits the following upward succession :— (6. Arenig beds . < 7 5: 4 / 3- Tremadoc beds \ 2. I. Volcanic ash and tuffs interstratified with blue-black fine slate. No fossils. Ty-obry beds. (Dark fine-grained ( bylina, Asaphus slates.) ) Calymene, Trinu- Thick band of grit dividing the two ) deus, Dionide,and series. (North Wales only.) ( Dendrograptus. Garth beds. (Dark slates with Asa- (Asaphus Homfrayi, phus, &c.) I Ogygia scutat-idx, J Angdina, Conu- < laria, and Dend- | roid Graptolites, I in the Arenig ' area. Lower Tremadoc. (Iron - stained J Niobe, Nesuretus slates and flags.) ( Conocoryphe, &c. Lingula flags. (Black slates with Oleni.) With the exception of the grit bed No. 4, the sequence in North Wales is the same as that of St. David’s, this divisional rock not occurring on the St. David’s Promontory.