Type:— U.S.A. Alaska: Prince William Sound, 1792, Menzies s.n. (holotype BM!). Fig. 2.

Caltha leptosepala var. rotundifolia Huth (1892: 68). C. rotundifolia (Huth) Greene (1899: 80). C. biflora var. rotundifolia (Huth) Hitchcock (1964: 335).

Type:— U.S.A. Rocky Mountains, 1872, A. Gray s.n. (isotype GH!).

Caltha chelidonii Greene (1899: 78).

Type:— CANADA. Alberta: Yellowhead Pass, Rocky Mountains on Alberta-British Columbia boundary, Jasper National Park, 13 July 1898, Spreadborough 19250 (holotype CAN!).

Caltha macounii Greene (1899: 77). C. leptosepala var. macounii (Greene) Davis (1900: 16).

Type:— CANADA. British Columbia: Mount Queest, 28 July 1889, Macoun 1255 (syntype US!).

Caltha leptosepala var. sulfurea Hitchcock (1964: 337).

Type:— U.S.A. Idaho: Custer County, Mount Borah, Rock Creek, 12 August 1944, Hitchcock & Muhlick 10942 (isotypes US! WTU!).

Morphology and cytology:—Leaf blades 1–1.4(–1.7) × as long as wide, +/- sagittate to cordate to obovate, auricles variable (diplophyllous or not, sinus open or closed), margins crenate or dentate (more so near base, tending toward entire near the apex) (Fig. 2); filaments broadly filiform to flattened, 0.2–0.7(–0.9) mm wide (Giblin et al. in press), slightly narrower to as wide as anthers (Figs. 2, 11C); pollen (3–)4–8(–12) colpate (rarely -porate) or misshapen (Fig. 9); follicles with curved stylar beak ~ 0.1 mm in length (Giblin et al. in press); allododecaploid (2 n = 12 x = 96, Wefferling et al. 2015; holoploid genome size ca. 18–25.5 pg/2 C, Wefferling et al. 2017), rarely allononaploid (2 n = 9 x = 72, Wefferling et al. 2015; holoploid genome size ca. 13.8–16.8 pg/2 C, Wefferling et al. 2017); apparently formed (bidirectionally) through hybridization of Caltha biflora and C. chionophila.

Taxonomic notes:— Candolle (1818) designated as holotype the Menzies collections from Prince William Sound. This part of the complex’s range is well outside the ranges of Caltha biflora or C. chionophila; all specimens that we have seen from north of Prince of Wales Island, Alaska (near Haida Gwaii, British Columbia) are apparently allododecaploids. In 1970, P. G. Smit annotated the holotype (Fig. 2), indicating that the pollen was colpate.

Geographic distribution:—Montane to subalpine in central Alaska and Yukon south through British Columbia to Olympic Mountain Range and Cascades of Washington and Oregon, Sierran California, east to western Alberta, central Idaho, northeastern and southeastern Oregon, and western Montana (Wefferling et al. 2017, Giblin et al. in press).

Additional specimens examined (*pollen sampled for SEM; n = 15; †ploidy level estimated via FCM or chromosome count [Wefferling et al. 2017], n = 33):— CANADA: Alberta: Sunshine Lodge, 51.131817, -115.565006, elevation 2286 m, Ogilvie, R. T. s.n., 19-Jul-1959 (V). British Columbia: Mount Revelstoke National Park: Along trail to Millar Lake, 51.066667, -118.1, elevation 1890 m, Soper, J. H. 12,754 with M. J. Shchepanek, 31-Jul-1970 (RBCM). British Columbia: Pan Creek, Ilgachuz Range, upper Pan Creek valley, 52.725, -125.225, elevation 1737 m, Hebda, R. 87-32, 27-Jul-1987 (RBCM). British Columbia: Tweedsmuir Provincial Park, Tweedsmuir Peak, 53.656667, - 126.501667, elevation 1717 m, Hebda, R and K. Marr KM4086, 9-Aug-2002 (RBCM). British Columbia: Green Mountain, Vancouver Island, 49.051134, -124.34007, elevation 1300 m, Marr, K., R. Hebda, L. Kennedy, and White 02-48*, 5-Jul-2002 (RBCM). British Columbia: Muskwa Ranges, Needham Creek, headwaters of creek, 56.4083, - 123.4989, elevation 1750 m, Hebda, R., K. Marr, and W. MacKenzie KM4777 †, 6-Aug-2003 (RBCM). British Columbia: Swannel Ranges, Chase Mountain, in valley bottom downstream from small lake 1 km S of summit, 56.558569, -125.255536, elevation 1700 m, Hebda, R. and R. Forsyth KM 4857m †, 7-Aug-2003 (RBCM). British Columbia: Rocky Mtn. foothills, Quintette Mtn., Roman Mtn. summit, 5 km WNW of Quintette Mtn, S of Babcock Creek, 54.888, -120.941, elevation 1650 m, Hebda, R., K. L. Marr, and R. Forsyth KM5613 †, 11-Aug-2003 (RBCM). British Columbia: Skeena Mountains, Klappan Mountain, small mountain 2 km W of Klappan Mtn. summit near road, 57.238333, -128.910001, elevation 1751 m, Marr, K. L., R. Hebda, and S. Berger 05-0652 †, 6-May-2006 (RBCM). British Columbia: Coast Mountains, Hanna Ridge, S end Hanna Ridge, N of Meziadin Lake, 56.223056, -129.436944, elevation 1752 m, Marr., K. L., R. Hebda, and W. MacKenzie 06-0013* †, 13-Jul-2006 (RBCM). British Columbia: Coast Mountains, Copper Mtn., mountain summit east of town of Terrace, 54.5117, -128.4536, elevation 1199 m, Marr., K. L., R. Hebda, and W. MacKenzie 06-0523 †, 18-Jul-2006 (RBCM). British Columbia: Stikine Ranges, Blue Sheep Lake, south of Little Blue Sheep Lake, 58.717113, -128.258892, elevation 1685 m, Marr, K. L., R. Hebda, and W MacKenzie 07-1648 †, 12-Aug-2007 (RBCM). U.S.A.: Alaska: Hatcher Pass, 61.783331, -149.266661, elevation 1200 m, Hawkins, L. L. s.n., 26-Jun-1976 (ALA). Alaska: Denali National Park and Preserve, Wildhorse Creek Valley, 5.9 km SW of toe of Kanikula Glacier, 7.3 km upstream of confluence of Wildhorse Creek and Tokositna River, Alaska Range, AK, 62.653, -150.961, elevation 838 m, Larsen, A. and M. B. Cook 01-0092 †, 2-Jul-2001 (ALA). Alaska: Fairweather Range, mountain E of Brady Glacier, N of Taylor Bay, 58.463567, -136.470102, elevation 360 m, Stratford, L. and J. Grunblatt LS01-7, 20-Jul-2001 (ALA). Alaska: Guyot Hills, 60.168667, -141.405991, elevation 933 m, Loomis, P. and A. Larsen 1256 †, 27-Jul-2003 (ALA). Alaska: Tongass NF, Alexander Archipelago, Baranof Island, near lake above Lake Diana, 56.8908, -135.045301, elevation 655 m, Calhoun, K. and B. Krieckhaus 37, 1-Jul-2004 (ALA). Alaska: Afognak Island, Kodiak National Wildlife Refuge, Blue Fox Bay, head of bay in vicinity of public use cabin, 58.413056, -152.696111, elevation 15 m, Parker, C. L. and S. Studebaker 17293 †, 2-Jul-2008 (ALA). Alaska: Kodiak Island, SE region of island, Big Creek Valley, 11 km NNE of Old Harbor and E side of valley, 57.29537, - 153.29202, elevation 685 m, Parker, C. L. 17964, 5-Aug-2010 (ALA). Alaska: Steep Creek Basin, near Juneau, 58.393812, -134.515948, elevation 585 m, Taylor, S. G. 55, 1-Jul-1968 (ALA). Alaska: Thompson Pass, Richardson Highway, 61.133256, -145.750058, elevation 900 m, Parker, C. L. 2368, 21-Jul-1990 (RSA). Alaska: St. Elias Mtns, Chilkat R. headwaters, 5km S of Klukwah Mt., 59.53, -135.83, elevation 910 m, Parker, C. L., A. R. Batten, and D. Blank 9523*, 16-Jul-2000 (UWM). Alaska: Chugach NF, Lost Lake trail from Seward side, 60.202889, -149.427667, elevation 390 m, Wefferling, K. M. 64 †, 15-Jun-2013 (UWM). Alaska: Chugach State Park, Williwaw Lakes trail, 61.112389, -149.660278, elevation 620 m, Wefferling, K. M. 65 †, 17-Jun-2013 (UWM). Alaska: Chugach NF, Falls Creek Trail, 61.003389, -149.572778, elevation 629 m, Wefferling, K. M. 63* †, 13-Jun-2013 (UWM). California: Lily Lake, W side of the Warner Mountains, Sierran montane forest, Fort Bidwell 15’ Quad., 41.976368, -120.202733, elevation 2130 m, Bartolomew, B. 4327 and B. Anderson *, 24-Jun-1988 (NYBG). Idaho: Bear Valley Road (FS579), Boise National Forest, old burned forest along small creek south of Bruce Meadows, 44.367061, -115.27463, elevation 2184 m, Smith, J. F. s.n. †, 27-Apr-2015 (SRP). Idaho: Salmon-Challis NF, Salmon River Mtns. Beaver Creek drainage, ca. 10 air mi. NE from Cape Horn Guard Station, 44.446347, -115.035753, elevation 2225 m, Tanaka, T. and C. Richardson 82* †, 10-Jul-1996 (ID). Idaho: Moses Butte area, near intersection of FR 457, FR 220 and FR 363, in meadow just to south and west of road, 47.0091, -115.8415, elevation 1875 m, Parks, M., L. Stratford, and R. McNeill 620* †, 15-Jul-2004 (ID). Idaho: Salmon River Mtns, Boise NF, Lowman RD, Canyon Creek at pullout along ID 21, 44.288372, -115.226992, elevation 2060 m, Wefferling, K. M. 180h †, 21-Jul-2014 (UWM). Idaho: Boise National Forest, Summit Lake, 44.649947, -115.583245, elevation 2280 m, Smith, J. F. 2962*, 26-Jun-1994 (SRP). Oregon: Mt. Hood Wilderness, NW end of Elk Meadows, 45.34525, -121.620528, elevation 1570 m, Wefferling, K. M. CR5 * †, 1- Jan-2015 (UWM). Montana: Lost Trail Bog, Lost Trail Pass, near continental divide, 45.693303, -113.953169, elevation 2146 m, Mantas, M. 585*, 5-Jul-1992 (ID). Oregon: Wallowa Whitman National Forest, Anthony Lakes area, streamside near trail from Anthony Lake to Hoffer Lake, 44.954083, -118.233472, elevation 2212 m, Wefferling, K. M. and L. Woo 62* †, 12-Aug-2012 (UWM). Oregon: Jefferson Park in the Cascade Mountains on the border of Marion county and Jefferson County, 44.712148, -121.797064, elevation 1790 m, Anderson, J. JNS2-1 * †, 1-Jan-2015 (UWM). Oregon: Mt. Hood Wilderness, NW end of Elk Meadows, 45.34525, -121.620528, elevation 1570 m, Wefferling, K. M. 67* †, 1-Jul-2013 (UWM). Oregon: Mt. Hood Wilderness, NW end of Elk Meadows, 45.34525, -121.620528, elevation 1570 m, Wefferling, K. M. 70 †, 1-Jul-2013 (UWM). Oregon: Fremont NF, three miles N of Dead Horse Lake, at junction of Bald Butte Road #450 and Road #3411, 42.601, -120.781, elevation 2073 m, Legler, B., S. Gage, W. Gibble, R. Goff, S. Birks, and K. Davis 1890 †, 2-Jul-2004 (WTU). Oregon: McCoy Creek on Steens Mountain, 42.725724, -118.597244, elevation 2344 m, Lowry II, P. P. 469*, 7-Jul-1976 (OSC). Washington: Mount Baker Nat’l Forest, Harts Pass, 48.72, -120.67, elevation 2000 m, Muenscher, W. C. 10065, 23-Jun-1939 (UWM). Washington: Gifford-Pinchot NF, Takhlakh Meadow, 46.269707, -121.588677, elevation 1400 m, Wefferling, K. M. 60 †, 1-Aug-2012 (UWM). Washington: Mount Rainier National Park. Spray Park, 46.944372, -121.750034, elevation 1892 m, Rochefort, R. SP4 †, 22-Jul-2015 (UWM). Washington: Wenatchee Mountains, 47.42, -120.94, elevation 1187 m, collector unknown [Washington Native Plant Society WNPS-2 † ], 1-Jan-2014 (UWM). Washington: Gifford-Pinchot NF, Takhlakh Meadow, 46.268447, - 121.586248, elevation 1400 m, Wefferling, K. M. and L. Wefferling 87s †, 15-Jul-2014 (UWM). Washington: Gifford-Pinchot NF, Chain of Lakes, 46.29321, -121.596996, elevation 1340 m, Wefferling, K. M. and L. Wefferling 118bh †, 16-Jul-2014 (UWM). Washington: Horseshoe Basin (HB00711), Arnold Peak, SW face, on bench with steep rocky springs, and open coniferous forest, 48.985377, -119.924039, elevation 2317 m, Wooten, G. # GW01566, 28-Jun-1992 (WTU). Washington: North Cascades National Park, Pelton Basin east of Cascade Pass, 48.458683, -121.0433, elevation 1407 m, Knoke, D. 267 †, 10-Aug-2002 (WTU). Washington: Okanogan NF, Washington Pass Viewpoint turnoff, wetland at Hwy. 20 entrance, 48.525081, -120.658038, elevation 1649 m, Yen, A. C. 03-066 with R. Robohm, S. Bagshaw, L. Van Volkenburgh, and T. Ohlson, 25-Jul-2003 (WTU). Washington: Okanogan NF, Meadows Campground, meadows S of Hart’s Pass, 48.711, -120.676, elevation 1903 m, Rodman, S., D. Tank, C. Spurgeon, and K. Ardern 909*, 25-Jul-2003 (WTU). Washington: Gifford-Pinchot NF, Takhlakh Meadow, 46.270167, -121.588028, elevation 1402 m, Wefferling, K. M. 56 †, 1-Aug-2012 (UWM). 9 x: U.S.A., Idaho: E side Sawtooth Range, Sawtooth NRA, Elk Meadows (NE side, edge of meadow/forest), 44.266698, -115.094952, elevation 2060 m, Wefferling, K. M. 212rh †, 22-Jul-2014 (UWM).