FIGURES 17‒22 in Cyclotella petenensis and Cyclotella cassandrae, two new fossil diatoms from Pleistocene sediments of Lake Petén-Itzá, Guatemala, Central America

FIGURES 17‒22. Type material of fossil lacustrine diatom Cyclotella petenensis from lake Peten Itza, SEM. (17) Internal view of the valve showing near the margins partially occluded alveoli by the central lamina, an arc of CFP (white arrowheads) consisting of one tube surrounded by three satellite pori in the central area and a single sessile RM (white double arrowhead) opposite to the arc. (18) Internal detailed view of the valve margin with MFP located on each costae and a stalked rimoportula with slightly deflected labium. (19) Internal view of costae-born MFP consisting of a tube surrounded by three satellite pores and the protruding RM with an oblique slit. (20) External view of the external openings of marginal fultoportula (white arrowhead) consisting of very short tubes separated by rows of very fine areolae and granules. (21) Detailed view of a broken valve margin showing a narrow canal connecting internal MFP to their external opening. (22) External view of two valves connected by a cingulum consisting of an open valvocopula (white arrowhead) and several copulae. Scale = 10μm (17, 22), 2μm (18–21). Abbreviations: central fultoportulae (CFP), marginal fultoportulae (MFP), rimoportula (RM).