Fossil calcareous leaf-like algae are common constituents of marine limestones of Pennsylvanian and Permian age. Not only are they important rock-building organisms, but their broken calcified remains were often piled up like corn flakes to form prolific reservoir rocks for petroleum in the Paradox basin of southeastern Utah and southwestern Colorado, in Kansas, and elsewhere worldwide. The collective term was introduced by Pray and Wray (1963, p. 209) to accommodate these fossils. Genera commonly included are the assumed codiacean Eugonophyllum Konishi and Wray, 1961, Anchicodium Johnson, 1946, Ivanovia Khvorova, 1946, and Calcifolium Schvetzov and Birini, 1935, and the rhodophycean genus Archaeolithophyllum Johnson, 1956 (Wray, 1977). However, as we will see later, Anchicodium is not a phylloid alga by definition. These genera are, by necessity, identified by their distinctive internal microstructure as seen in thin section; none have been seen, or at least documented, in growth position. Their living morphologies have been inferred entirely on individual interpretation. Unfortunately, the abundant fossils occur only as broken fragments, generally in a poor state of internal preservation. Consequently, usage of generic names has become more territorial than realistic. For example, it was determined in the late 1950's that the fossil fragments that comprise the prolific petroleum reservoir rocks in the Paradox basin were of the genus Ivanovia. When reasonably well preserved, Ivanovia has a distinctive internal structure consisting of two parallel walls of that form regular rectangular-shaped dark, microcrystalline calcite interutricular deposits, separated by light-colored sparry calcite that has infilled the areas formerly occupied by the (Baars, 1968; Wilbur et al., 1969; Torres and Baars, in press). No medullary siphonaceous filaments have been found, or reported, between the wall structures; the space is invariably filled with sparry calcite. By far the most common occurrences of plateshaped fragments consist entirely of crystalline calcite with no microstructure preserved; since the genus Ivanovia is known to occur in the Paradox basin, all such featureless fragments are commonly referred to as Ivanovia in rocks of Middle Pennsylvanian age. The same is true for the genus Eugonophyllum in rocks of Upper Pennsylvanian and Lower Permian age in southern New Mexico and West Texas. This apparently phylogenetically advanced genus has loop-shaped interutricular deposits forming regular wall structures along either opposing wall of the thallus (Konishi and Wray, 1961). Only recently, Kirkland et al. (1991) have reported the presence of medullary filaments in Eugonophyllum. These, too, most commonly occur as blades of clear sparry calcite with no visible internal structure, but are commonly referred to as Eugonophyllum in that region. Toomey and Babcock (1983) discussed the Genus Eugonophyllum, a relatively well known phylloid alga. They noted that the genus had the growth habit of an erect plant, up to 5 inches in height. The plant consisted of relatively broad potato-chiplike leaves, which could not have provided a substantial reef framework. They found only fragmented leaves; neither the holdfasts nor stalks were retrieved. There is bilateral symmetry of the plant's internal structure, particularly the of the outer layers of opposing walls. It appears that only the outer edge of the algal thalli were carbonate encrusted, since in thin section the internal portion of the algal blade is almost invariably recrystallized to sparry calcite mosaic. Rare finds of silicified phylloid algal plates have been made, and these occurrences seem to suggest that when the plants died, the 'leaves' fell to the seafloor where they tended to accumulate in piles, somewhat analogous to piles of dead leaves on a forest floor (Toomey and Babcock, 1983, p. 131). This rather graphically describes the nature of the genus. However, the dark pillar-like utricles of the wall structure are actually interutricular deposits precipitated between the former utricles, exterior to the cell walls, similar to those of Ivanovia discussed above. The general description could be applied to the genus Ivanovia as well. The principle difference between the two genera being the shape and position of the interutricular deposits; those of Eugonophyllum being somewhat recessed from the edge of the thallus and more rounded, with those of Ivanovia being marginal and rectangular in outline. Both genera are double-walled, with opposing identical interutricular structures. Neither genus has been found with preserved medullary structures, with the unique exception described by Kirkland, et al., (1991) nor in growth position. Phylloid algae of the Midcontinent U.S.A., particularly the moundbuilding organisms in Kansas, have mistakenly been identified as Anchicodium (Konishi and Wray, 1961; LaPorte, 1962; Crowley, 1969). The fossils are generally characteristic of other phylloids, but are larger and