IX.—The Placoderm Fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Old Red Sandstone of Scotland. Part I. Descriptive Morphology

Coccosteus cuspidatus is shown to be the correct name for the arthrodire commonly referred to as Coccosteus decipiens. An almost complete account of the osteology of this species is given and aspects of the restored head, thoracic armour, fins and posterior unarmoured region, the dermal ornamentation, neuromast system and musculature are discussed; the snout, dermal bones of the palate and neural endocranium receive special treatment in view of the part these structures have played in the interpretation of the relationships of the Arthrodira. Changes in the proportions of certain bones during growth are noted and the mode of life is considered. The family Coccosteidae is redefined; the genus Coccosteus and the type species C. cuspidatus are defined and consideration is given to other species included in this genus