5. R. arvénsis, Aust. Thallus much divided, 3–9´´ broad, papillose-reticulate, dull green both sides, becoming fuscous above, the flat margins at length purple; divisions sulcate, dichotomous, the linear-elliptic or subspatulate lobes acutish and obsoletely emarginate; cilia whitish, very short and inconspicuous or nearly wanting; capsules aggregated toward the apex; spores 71–84 µ broad, dark fuscous, reticulate, with pellucid margin.—Var. hírta, Aust., decidedly ciliate and with spine-like hairs scattered over the upper surface; divisions broader, more obtuse; spores nearly black, larger (84–101 µ) and smoother.—Cultivated fields and (the var.) rocky places, Closter, N. J. (Austin).
6. R. Lescuriàna, Aust. Stellately or subcruciately divided, the obcordate or cuneate-linear divisions 2–6´´ long, punctate-reticulate, subglaucous or ashy-green both sides or becoming purple beneath, the slightly concave lobes emarginate, closely ciliate with short obtuse spine-like white hairs; capsules scattered, chiefly near the base of the divisions; spores 71–83 µ broad, dark brown, reticulate, not margined.—Cultivated fields and rocky grounds, N. J. to Ill., and southward.
§ 2. SPONGÒDES. Thallus with large air-cavities usually opening by pits through the upper surface, and with slight depressions over the capsules which are prominent beneath; spores 41–51 µ broad, obtusely angular or globose; terrestrial.
7. R. crystállina, L. Orbicular, 6–9´´ broad, the obcordate or cuneate divisions bifid or 2-lobed, flat above and the surface much broken up into pits, the margins subcrenate; capsules scattered; spores issuing through the upper surface.—On mud flats, Ill. (Hall), west and southward. (Eu.)
8. R. lutéscens, Schwein. Orbicular, 1–1½´ in diameter, light green; divisions 6–8, linear, 2–3 times forked, narrowly channelled, obcordate and thickened at the apex, with delicate whitish obliquely ovate appressed scales, rootlets wanting beneath above the middle; fruiting plant unknown.—Dried up pools and ditches, Canada to Mo., and southward. An analogous form has been developed by Lindberg from R. natans.
9. R. ténuis, Aust. Thallus thin, olive or yellowish-green, shining, the 2–4 divisions roundish-obovate, 2–4´´ long, flat, with sinuate margins, green beneath with a slender costa and few rootlets; capsule very delicate, closely adherent to the substance of the thallus, minutely apiculate; spores round or short-oval, conspicuously depressed at one end when dry.—Wet ground in open woods, Closter and Lawrence, N. J. (Austin), and Mo. (Hall).
§ 3. RICCIÉLLA. Thallus linear, dichotomous, floating or rarely terrestrial; capsule protuberant from the lower surface.
10. R. flùitans, L. Thallus often in extended patches, thin, green, radiately expanding, the often imbricate divisions ½–1½´´ wide, parallel-nerved, flat, without rootlets, cavernous only toward the slightly dilated very obtuse or subtruncate apex; capsules present only in some terrestrial forms, very prominent below, rupturing beneath the apex.—Very variable. The most notable form is var. Sullivánti, Aust., with divisions about ½´´ wide, channelled, cavernous throughout, the margins crisped-crenulate, and rootlets numerous on the costa tumid with abundant capsules, which are tipped with a long funnel-mouthed point; spores obscurely angled, reticulate and margined. (R. Sullivanti, Aust.)—In ponds or ditches or growing in wet places upon the ground; the variety often in cultivated fields. (Eu.)
§ 4. RICCIOCÁRPUS. Thallus obcordate, floating or rarely terrestrial; capsules not protruding, at length exposed by a cleft in the central groove.
11. R. nàtans, L. ([Pl. 22.]) Divisions obcordate or cuneate, broadly emarginate, 3–6´´ long, purplish, very narrowly channelled, with numerous uniform air-cavities beneath the epidermis, rooting toward the base and at length with dark purple scales beneath the apex; capsules in 1 or 2 rows beneath the groove; spores black, angular, strongly papillose.—Canada to the Gulf. (Eu.)