A taxonomic revision of some diatom species of the genera Biddulphia Gray and Triceratium Ehrenberg is made. The species are: B. azorica Pavillard, B. schroederiana Schussnig, B. membranacea Cleve. T. formosum Brightwell f.formosum, T. formosum f. quadrangularis (Hustedt), T. formosum f. quinquelobulata (Hustedt), T. shadboltianum Greville, T. shadbottianum f. elongata (Grunow) Hustedt and T. pelagicum (Schroder) Sournia. These species have poor siliciflcation of the frustules, reduced valvar apendixes and spines or none; these features could be regarded as adaptations to pelagic life. Descriptions and microphotographies of the species were made. These Biddulphia spp were found to be very close and some morphometric differences in the relation apical axe/pervalvar axe were taken in order to identify them quickly. All forms were found to be more frequent in no rain season and in high salinity waters (36 ‰). For T. formosum f. quadrangularis (Hustedt) and T. formosum f. quinquelobulata (Hustedt) some explanations were made about its real infrasnecific position, so that T. quadrangulare Greville and T. quinquelobutarum Greville should be regarded as different species of that forms of T.formosum Brightwell. B. azorica Pavillard and B. schroederiana Schussnig are first reported for the Caribbean Sea area.
