Plants small to moderate. Stem solitary, erect, ± smooth. Leaves: sheath producing soft fibers; petiole split at base, unarmed; abaxial hastula a small crescent-shaped ridge or absent; adaxial hastula irregularly semicircular to nearly cylindric; blade palmate; plication induplicate; segments lanceolate, basally connate; cross veins conspicuous [obscure]. Inflorescences interfoliar, emerging through split leaf bases, arching beyond leaves, with 2 orders of pendent branches; prophyll short; peduncular bracts many, tubular. Flowers bisexual, borne singly along rachillae, short- to long-pedicellate; perianth 1-seriate, shallowly cupulate, lobes 5--7, apiculate; stamens 6--12, erect [inflexed in bud]; filaments acute; anthers dorsifixed; pistils 1, 1-carpellate, glabrous; stigma funnelform. Fruits globose; stigmatic scar apical; exocarp white, smooth, slightly warty or rugose when dry; mesocarp mealy; endocarp membranaceous. Seeds oblate-globose; hilum deeply intruded into seed, forming cylindric depression or perforation; endosperm homogeneous, bony; embryo nearly apical; eophyll undivided, lanceolate. nx = 18.

Thrinax lacks a showy perianth and is wind pollinated, although many insects visit the inflorescence (D. W. Roubik, pers. comm.) and may transferport some pollen once the lateral lobes of the stigma have opened to expose the stigmatic surfaces. Fruits are taken by birds (R. W. Read 1975), including the red-bellied woodpecker (Melanerpes carolinus), lizards (J. B. Iverson 1979), and gray squirrels, which probably disperse the seeds. Key deer consume T. morrisii fruits, but they are probably not seed dispersers (W. D. Klimstra and A. L. Dooley 1990; W. D. Klimstra, pers. comm.).