Heladena sect. Hassleria Nied., Arbeiten Bot. Inst. Königl. Lyceum Hosianum Braunsberg 5: 15. 1914. —Type: H. hassleriana Nied. [C. chlorocarpus (Adr. Juss.) W. R. Anderson].
Woody vines. Petiole eglandular or bearing 2–4 (–6) small glands; lamina eglandular or bearing 2–many tiny glands on or embedded in margin; stipules very small, triangular, borne on stem between petioles or beside base of petiole, caducous or persistent. Inflorescences axillary or terminal, pseudoracemes or umbels or panicles of pseudoracemes or umbels, the flowers mostly decussate; bracts and bracteoles small, eglandular, persistent; floriferous peduncle often well developed, reduced or lacking in some populations. Sepals leaving petals exposed during enlargement of bud, appressed in anthesis, all 5 eglandular or the lateral 4 bearing large paired abaxial glands; corolla bilaterally symmetrical, the posterior petal erect and ± different from the spreading lateral 4; petals yellow, abaxially densely sericeous on claw and limb except near margin, adaxially glabrous; androecium radially or bilaterally symmetrical; stamens 10, all fertile; filaments connate in proximal 1/4–1/2, sometimes subequal but mostly of differing lengths, often longest opposite anterior sepal and shortest opposite posterior petal, the 2 opposite posterior-lateral petals often stouter than others; anthers alike or subequal, glabrous or sparsely hairy; pollen globally symmetrical, with colpi not perpendicular to the equatorial plane and pores variously distributed; receptacle glabrous on both sides of stamens; gynoecium radially symmetrical, comprising 3 carpels, all fertile, connate their whole length in ovary; styles 3, erect and straight or somewhat bowed or spreading, alike, dorsally rounded to short-apiculate or short-hooked and laterally compressed or terete at apex, the stigma internal. Fruit dry, breaking apart into samaras separating from a high pyramidal torus; samara butterfly-shaped with lateral wings dominant, chartaceous with many fine parallel veins, cleft to nut at base and apex, the margin sinuate to coarsely dentate; dorsal wing(s) lacking or, if developed, small, distinct at base and apex; ventral areole ovate to linear; samara reduced or highly modified in some populations of C. sinemariensis; carpophore absent. Chromosome number unknown.
Six species of Mexico, Central America, the Lesser Antilles, and South America. The species of Carolus grow in diverse habitats, including wet forests, dry forests, second-growth woods, thickets in dry, often rocky, places, and savannahs. [map] — Regional key to genera: Caribbean, Central America.
Carolus is distinguished by its interpetiolar stipules, marginal leaf glands, densely sericeous yellow petals, and butterfly-shaped samaras, with the lateral wings chartaceous and containing many fine parallel veins. Four of the six species (C. anderssonii, C. chasei, C. chlorocarpus, and C. sinemariensis) are included in the latest sampling of DNA sequences (Davis & Anderson, 2010 [pdf]), and the bootstrap support for the genus is 100%. Because they both have interpetiolar stipules and hairy yellow petals, this genus can be confused with Amorimia, but that genus differs in having its leaf glands usually in the abaxial surface between the midrib and the margin, and in its large spreading gland-bearing bracts.
Reference: W. R. Anderson (2006b, pp. 186–189), partial revision: description of genus, key to species, nomenclature of species, but no descriptions of species (Carolus only: pdf; entire paper: pdf); unpublished notes on species of Carolus.