Since 1876, when McCoy described two specimens of tuberculated plates under the name Asterolepis ornata var. australis, placoderm remains have been known to occur in the Middle Devonian marine limestones of Buchan, Victoria, Australia. Some years later, another specimen was obtained from the Buchan limestones by W. H. Ferguson of the Geological Survey of Victoria, and was referred by Chapman (1916) to the genus Phlyctaenaspis. Chapman, regarding McCoy's specimens as cospecific with the new material, included all the remains in the species Phlyctaenaspis australis (McCoy), distinguishing the more complete specimen discovered by Ferguson as P. australis var. confertituberculata Chapman. As Phlyctaenaspis is a form typical of the Lower Devonian, its reported occurrence in the Middle Devonian seemed to warrant further investigation. I therefore undertook a re-examination of all the available material, which is now preserved in the National Museum, Melbourne, and spent some months in removing the matrix from Ferguson's specimen. The matrix is a hard carbonaceous limestone containing remains of Spirifer yassensis and other invertebrates, and it was found that dilute hydrochloric acid would dissolve away this matrix without seriously affecting the fish plates, although care had to be taken not to subject these to too long contact with the acid. When as much of the matrix as possible had been removed mechanically by grinding, the fossil was exposed by brushing the dilute acid on to the remainder with a camel-hair brush, and continually observing the result under a binocular microscope.