Benthic marine diatom flora of Guam: new records, redescription of Psammodictyon pustulatum n. comb., n. stat., and three new species (Colliculoamphora gabgabensis, Lauderia excentrica, and Rhoiconeis pagoensis) Author Lobban, Christopher S. Division of Natural Sciences, University of Guam, Mangilao, GU 83823, USA e-mail: clobban @ guam. net clobban@guam.net text Micronesica 2015 2015 2 1 49 journal article 299054 10.5281/zenodo.12117576 9ec266f5-1dc7-4272-9fbc-67c7f2b0f5aa 0026-279X 12117576 Mastogloia seychellensis Grunow Figs 80–82 Section Paradoxae Ref. illus.: Hustedt 1931 –1959, fig. 958; Loir & Novarino 2013 , pl. 21c Samples: GU52Q-10a Dimensions: Length 45–48 µm, width 13 µm; transapical striae 27 in 10 µm, longitudinal striae 26 in 10 µm. Diagnostics: Elliptical-lanceolate valves with rostrate apices, the partecta strongly displaced toward the midline and with evident ducts leading obliquely to the valvocopula wall. Comments: M. seychellensis is similar to M. paradoxa Grunow ( Hustedt 1931 –1959, fig. 953; in our flora: Lobban et al. 2012 , pl. 34, figs 3–5) and M. similis Hustedt ( Hustedt 1931 –1959, fig. 954); the latter differs from M. paradoxa only in having a straight raphe. In these taxa the middle chambers are distinctly larger than the rest, instead of forming a graded series as in M. seychellensis , and both have axial costae along the raphe, which are absent from M. seychellensis . The size ranges of M. seychellensis and M. paradoxa are respectively 30–50 µm and 50–60 µm according to Hustedt. Although Hustedt showed the raphe of M. seychellensis as perfectly straight, the Guam specimens have a distinctly wavy raphe. On the strength of the difference Hustedt (1931 –1959) used to erect M. similis , this would be enough to exclude these specimens from M. seychellensis but we do not yet have adequate material to address the differences among these species.