A new centric diatom genus, Ferocia Van de Vijver and Houk gen. nov. is described based on Melosira setosa Greville. Several samples collected in lava tunnels on Ile Amsterdam, a small isolated, volcanic island in the southern Indian Ocean where the latter taxon dominated the diatom flora, were investigated. The new genus is characterized by having heavily silicified, spherical frustules, connected into long chains by large, complex linking spines. Valves are dome-shaped with a high mantle with rounded to irregularly shaped areolae. Numerous small rimoportulae forming a marginal ring are present near the mantle edge. The cingulum is composed of a large number of narrow, open copulae. A second species, Ferocia ninae Van de Vijver and Houk sp. nov., is described from the nearby Crozet archipelago. Besides the typical features of the genus Ferocia, the rimoportulae in F. ninae are almost equidistant, the valves have a relatively low valve height and the spines are relatively small. Both F. ninae and F. setosa were found in shaded lava tunnels with a sparse moss vegetation. The two species are illustrated and discussed based on detailed LM and SEM observations.