The task of examining and determining the fine series of lamellibranchs collected by Mrs R. Gray in the Girvan district, from rocks of Llandeilo, Bala, and Llandovery age, has been no light one. In the first place, the literature of the subject is very scattered, and is chiefly to be found in the various reports of the Geological Surveys of the different States of North America and Canada. The work of British palæontologists is to be found in the publications of the Geological Survey and in various periodicals, but nothing systematic has been attempted before in this country, although Sowerby described and figured, in an appendix to Murchison's Silurian System, all the known species. Under these circumstances there has been the greatest difficulty in studying original types, many of which are Swedish or North American.