The specimens which form the material on which this paper is based were found by me in the flag fences on the farm of Lybster in the parish of Reay. Owing to various difficulties in growing hedges, such as exposure to severe storms from the Atlantic Ocean and a very cold subsoil, farmers and estate officials have recourse to the flagstones which are almost invariably found immediately beneath the surface, especially in the north-western portion of the county. If it happens that a fossiliferous bed is opened out, then careful searching will often reveal some of the fish-remains for which the Lower Old Red is famous. The commonest beds which yield fossil-fishes to any great extent are the Osteolepis beds, which are of a light chocolate colour, weathering to a bluish white. These beds, however, are too soft for fencing purposes. Consequently one need never expect to find in the fences fishes characteristic of these beds. Thus, it comes about that the scales, teeth, plates, and head-bones that are to be found, are characteristic of fishes that occur in the beds where fossil-remains are found only to a somewhat scanty extent. They are representative, therefore, of rarer fishes, and are worthy of more than usual attention. Numerous spines occur in the Lower Old Red as developed in the neighbourhood of Thurso. The great majority of these, however, are at once seen to be Acanthodian in nature and characteristic of the genera Mesacanthus and Diplacanthus. It was accordingly a matter of