Trigonium arcticum and Trigonium formosum, marine diatoms epiphytic on Ectocarpus sp., were collected from the sublittoral off Point Loma, California. Forming valves of these two sympatric species were studied using scanning electron microscopy. The earliest developing valve of Trigonium arcticum encountered is a porous basal layer of silica, triangular, with pseudocelli just starting to form at each corner. The central annulus, the site of the earliest valve formation, appears to be about the size of a single areola. Early rimoportulae (labiate processes) are interspersed between central areolae and are simple oval openings. Internally these form their labiate lip-like structures just before cribrum formation. Ridges form on the external side of the basal layer in a honeycomb pattern and produce the walls of the loculate structures. The ridges expand outward until cribrum formation is initiated. Small protuberances grow out from the edge of the ridge-like structures and then become anvil-shaped as they grow toward the center of the pores and form the "roofs" of the loculi. In external view of the mature valve, the pores are smoothly covered over except for a ring of C-shaped slits. These cribra are flush with the surface of the valve. At about the time the cribra are completing, rotae are formed within the porelli of the pseudocelli by extensions from two or three sides. The quadrangular Trigonium formosum develops along much the same pattern. Differences are the less numerous, more centrally located rimoportulae, which possess external tubes and the formation of highly domed cribra.

Key words: diatom, epiphyte, Trigonium arcticum, Trigonium formosum