The fauna of the Derfel limestone of the Arenig District of North Wales

The Derfel Limestone, a thin basal member of the Lower Bala of the Arenig-Bala district, north Wales, is exposed only in three localities but has yielded a rich fauna of brachiopods, trilobites, bryozoans and crinoids. A systematic study of the two former phyla shows that the brachiopods are represented by eighteen genera (one, Salopia, is described as new), including nine new species and subspecies, and the trilobites by nine genera. It has previously been believed that the fauna represented a Scoto-Appalachian intrusion into the independent Anglo-Welsh faunal province. But the brachiopods especially are not typical of contemporary Scottish or American sedentary benthos and are better described as a native association dominated by migrants from the east Baltic together with an association of clitambonitids, leptestiinids and productorthinids possibly of Russian origin.