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Several new broad-leaved monocots were identified in gray siltstones and fine-grained sandstones from the Upper Cretaceous (Campanian-Maastrichtian) St. Mary River Formation near Cardston, Alberta, Canada. Specimens are compression/impressions of long-petiolate aquatic plants that were probably buried in situ. Leaf blades are entire, ovate to elliptic, with deeply cordate bases; leaf blades are 5-12 cm long and 3.5-8.5 cm wide. Petioles are at least 4 mm wide with five to seven primary veins that enter the leaf blade. Venation is campylodromous, resulting in 23-27 primary veins with three medial veins that remain unbranched to the apex. Major and minor secondary veins (ABAB pattern) diverge at angles of 45°-65° near the midveins and 900 near the leaf margin. Tertiary veins are usually unbranched, but occasional dichotomies and anastomoses occur. No freely ending veinlets are visible. The Cardston specimens are compared with extant leaves of Alismatales and show closest similarities to those of Lirnnocharis L., Hydrocleys Rich., and Butomopsis Kunth. These leaves are also similar to fossil leaves of Haemanthophyllum Budantsev, in particular H. cordatum Golovneva from the Maastrichtian-Danian deposits of the Koryak Highlands, Russia. A reexamination of the genus Haemanthophyllum, the generitype (H. kamtschaticum Budantsev), the holotype for H. cordatum, and the Cardston specimens results in the description of a new genus, Cardstonia tolmanii gen. et sp. nov. (Limnocharitaceae). This study points to the need for reexamination of the remaining species in the genus Haemanthophyllum, which appear to represent a diverse assemblage of leaves of varying morphology that are probably not a natural group. [ABSTRACT FROM AUTHOR]