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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 unbranched 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 iLm), 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. There have been few studies of fossil freshwater diatoms of Guatemala, although numerous diatomaceous lacustrine deposits exist (Serrano, 1960). Newhall (1987) identified diatom deposits in the region around Lago de Atitlan, a tectonically and volcanically active region. We are not aware of any previous studies of the diatoms of these deposits by taxonomic specialists. Because the deposits are associated with rocks of known age, the associated diatoms can be used to test and/or refine a proposed biochronology of freshwater centric diatoms (Krebs et al., 1987). This paper reports two new extinct species of the genus Cyclostephanos Round from some of the younger diatom deposits in the Atitlan basin. A second objective of this paper is to evaluate different techniques for recording costa abundance in centric diatoms. Density of costae (or its equivalent, fascicle density) is traditionally used. However, costa density has never been shown to be superior to any other estimate of costa abundance in taxonomic studies. Two estimates of costa abundance, density, and total number are used I We thank Christopher Newhall for the diatom sample from Godinez and for information on radiometric dates. Kathryn Dieterich-Rurup and Gail Emmert provided technical assistance in preparing this manuscript. This investigation was supported by NSF grant BSR 8507709 to E.T. 2 Present address: Academy of Natural Sciences of Philadelphia, 19th and The Parkway, Philadelphia, Pennsylvania 19103, U.S.A. TRANS. AM. MICROSC. Soc., 108(1): 74-86. 1989. ? Copyright, 1989, by the American Microscopical Society, Inc. VOL. 108, NO. 1, JANUARY 1989 in a comparison of three fossil species of Cyclostephanos to assess the relative taxonomic effectiveness of each. MATERIALS AND METHODS Material containing the two new species was collected from fossil deposits at two localities in the western highlands of Guatemala in the vicinity of Lago de Atitlan. The first collection, called Rio de los Idolos, is from a roadcut about 5 km NW of the town of Patzun, Provincia de Chimaltenango, Guatemala (14?42.5'N, 91?2.5'W). This material was collected by Bradbury (Bradbury locality number 1 VII 75-311). The second collection, called Godinez, was taken about 1 km E of the town of Godinez, Provincia de Solola, Guatemala (14?42.5'N, 91?5.5'W). This material was collected by C. G. Newhall (USGS) and is assigned Bradbury locality number 2 XI 79-1. Although the collections are from the same general vicinity (about 5 km apart), their precise stratigraphic and geologic relationships are unknown. The diatomite at the Godinez locality is overlain by volcanic rocks radiometrically dated at 0.1 million years before present (Mybp) and is underlain by an ash flow tuff (Newhall, 1987). Apparently, this deposit is not a caldera deposit but represents local damming of a paleodrainage system that predated the Atitlan III caldera. Volcanic rocks beneath the diatomite are not well dated; they are believed to be less than 1 Mybp (Newhall, 1987). The age of the Rio de los idolos locality is not known, but is clearly younger than 3.3 Mybp, a radiometric date for a volcanic tuff underlying the deposit. Presumably, the Rio de los Idolos deposit also represents local damming of a paleodrainage system by volcanic flows and/or associated faulting. Cover slips with cleaned and dried material were embedded in Hyrax? for light microscopy (LM) or attached to aluminum stubs for scanning electron microscopy (SEM). LM observations were made on an Olympus Vanox? fitted with interference contrast optics. SEM observations on USGS 1 VII 75-311 were made with an ISI-DS 130 operated at 12-15 kV accelerating voltage. SEM observations on USGS 2 XI 79-1 were made on a JEOLCO 35C operated at 20 kV. Diameter and the total number of costae per valve were measured on each valve encountered until at least one representative of each 5-,um diameter size class was recorded. For 10 valves of each species, we measured areolar density in a stria over 5 ,tm because the valve face of both species is strongly undulate. We then doubled areolae per 5 Am as an estimate of the more traditional number of areolae in 10 gim. We made the same measurements on isotypic material of C. fenestratus Theriot & Kociolek for comparative purposes (Great Lakes Research Division ECT 708; USGS 3 IV 86-1; other slides are listed in Theriot & Kociolek, 1986). Terminology of diatom morphology conforms to Ross et al. (1979), with the exception of the term "lamina," a thin siliceous covering of the alveolus or marginal chamber, first used in this sense by Lowe (1975). 75 TRANS. AM. MICROSC. SOC.

Cyclostephanos lacrimis n. sp. and Cyclostephanos guatemalae n. sp.: Two New Centric Diatoms from the Fossil Record of Guatemala