This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Scapose from running rootstocks, 3'-6' high. Basal leaves long-petioled, the blade 1'-1 1/2' broad, reniform, 3-parted, the divisions broadly obovate, cuneate, obtuse, crenate or lobed; scape 1-flowered, slender, longer than the leaves, occasionally bearing a single deeply lobed leaf; flower 3"-5" broad, yellow; petals 5-6, veined with orange; sepals generally fewer and re-flexed; head of fruit globose, 6" broad; achenes flattened, ovate, gradually narrowed into a slender hooked beak.
R. pusillus Poir. in Lam. Encycl. 6: 99. 1804.
Annual, slender, weak, glabrous, branching, 6'-18' long. Leaves entire or denticulate, the lower oblong or ovate, sometimes cordate, on long petioles, the upper narrower, lanceolate or linear, short-petioled or sessile; flowers yellow, 2"-3" broad, the petals few, often barely exceeding the sepals; stamens 1-10; head of fruit globose, 2" broad; achenes beakless, merely tipped by the very short persistent style-base.
Marshes, southern New York and New Jersey near the coast, southeastern Pennsylvania, south to Florida and west through the Gulf States to Texas, north to Tennessee and Missouri. Dwarf crowfoot. April-July.
Ranunculus oblongifolius Ell. Bot. S. C. & Ga. 2: 58.
1821-24. Ranunculus pusillus var. oblongifolius T. & G. Fl.
N. A. I: 16. 1838.
Annual, branched above, erect or ascending, 1°-2° high, glabrous or pubescent. Leaves entire or denticulate, oblong, or oblong-lanceolate, the lower on long petioles, the blade sometimes 3' long, the upper narrower, lanceolate or linear; flowers yellow, 3,"-6" broad, the 5 petals much exceeding the sepals; stamens numerous; head of fruit 2" broad; achenes merely tipped by the very short style-base.
Ranunculus reptans L. Sp. Pl. 549. 1753. Ranunculus filiformis Michx. Fl. Bor. Am. 1: 320. 1803. Ranunculus Flammula var. reptans E. Meyer. Pl. Lab. 96. 1830. R. Flammula intermedins Hook. Fl. Bor. Am. 1: 11. 1829.
Trailing or reclining, glabrous or pubescent, rooting from the nodes, the flowering stems and peduncles ascending. Leaves linear, lanceolate or spatulate, 1'-2' long, mainly entire, gradually narrowed into the petiole; flowers bright yellow, 4"-5" broad, solitary on peduncles 1'-3' long, petals 4-7, much exceeding the calyx; achenes flattish, with a minute sharp beak; stamens numerous.
Ranunculus Flammula L., which has larger mostly broader leaves, the stout stem rooting only at the lower nodes, is recorded from Newfoundland, and is widely distributed in Europe and Asia.