Note: Both these taxa have been lumped together as Caltha leptosepala. I kept them separate just to aid their identification.
Broadleaved Marsh-marigold, Broad-leaved Caltha, Elkslip, Marsh Marigold, Twinflowered Marsh-marigold, White Marsh-marigold: Caltha biflora (Synonyms: Caltha biflora var. biflora, Caltha howellii, Caltha leptosepala ssp. biflora, Caltha leptosepala ssp. howellii, Caltha leptosepala var. biflora) - Rounded leaves about as long as wide.
Elkslip, Elkslip Marsh-marigold: Caltha leptosepala (Synoyms: Caltha biflora var. rotundifolia, Caltha leptosepala var. rotundifolia, Psychropila leptosepala) - Oblong leaves, longer than wide.