This is a Hemidiscus valve face, external surface. This image came from JGOFS Benthic Cruise TT013, horizontal net haul at 9N, 140W, JD 340. This diatom is a well known member of the warm water Pacific diatom plankton. They are heavily silicified species seen in many sediment samples, thus they contribute to the carbon and silica "expressway" from the surface to the seafloor.