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Two new species of Cyclostephanos are described from Pleistocene diatomites near Lago de Atitlan, Guatemala. In both species, radiating costae continue un- branched to the valve margin and the external expression of the single labiate process is an elongate pore on the mantle beneath a spine. Cyclostephanos lacrimis n. sp. is 7-75 jum in diameter and has no central area strutted processes. Each mantle strutted process is located within a marginal alveolus directly adjacent to a costa; this unique location is usually visible with light microscopy. C. guatemalae n. sp. is smaller (5-23 μm), with a single eccentrically-located strutted process and mantle strutted processes directly on every second or third costae. The best way to identify the two species on the basis of measured characters is to plot the total number of costae against diameter. Such a plot will almost completely separate the two species with slight overlap only among the smallest specimens of the two species. Costa density and valve diameter, often implicitly assumed to be independent characters, are found to have a strong curvilinear relationship in these two species and in C. fenestratus.