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During a survey of the coral reef diatoms of Moorea Island ( Society Archipelago, South Pacific) a small-sized member of the order Achnanthales was studied using a light microscope ( LM) and a scanning electron microscope ( SEM). This marine taxon has: a raphe valve ( RV) with a non-crenulate edge; a high cingulum; a sternum valve ( SV) often irregularly striated and areolae with concave hymenate pore occlusion; a thick and plain SV valvocopula ( SVVC), ring-shaped, composed of large fused fimbriae, with a central elliptic foramen bordered by the peg-like edge of the fimbriae. On abvalvar side, the SVVC bears radiate concave and robust transapical ribs, interlinking with short elevated transverse ribs of the RV valvocopula ( RVVC). Large marginal fenestrae of the RVVC give access to pseudoloculi. One oblong, unique and striated papilla is located on each RVVC rib. Given this unique set of features, we describe Xenococconeis opunohusiensis gen. et sp. nov. as a new taxon belonging to the Achnanthales. The characteristics of the new taxon are compared with Campyloneis Grunow and Cocconeis Ehrenberg. From New Caledonia, Cocconeis neocaledonica Maillard ex Lange- Bertalot et Steindorf, a freshwater diatom, was described with two internal septa with marginal pseudoloculi. Based on subsequent SEM illustrations and remarks, we propose the transfer of C. neocaledonica to the new genus, and compare it to the type species, Xenococconeis opunohusiensis. [ABSTRACT FROM AUTHOR]