Descripción: Plantas trepadoras o lianas con tronco marrón o rojizo. Ramitas con savia lechosa cortarlas. Hojas simples y opuestas, de 4-8 x 1-3 cm, elípticas o angostamente elípticas, con ápice agudo o acuminado, bordes enteros y base aguda. Las hojas son glabras. Flores blancas y pequeñas. Frutos en folículos, de 20-30 x 0.2-0.3 cm, glabros.

Especies Parecidas: A menudo se confunde con LK forsac Forsteronia acouci LK2 y LK forsmy Forsteronia myriantha LK2 , pero en F. acouci y F. myriantha las hojas tienen glándulas en la base de la nervadura central del haz, y en P. peninsularis las hojas no tienen glándulas en la base de la nervadura central del haz.

Liana, +/- glabrous; stems slender, usually less than 1 cm diam; outer bark thin, brown; sap thick, grayish-white. Leaves opposite; stipules inconspicuous; petioles 4-5 mm long, canaliculate; blades narrowly elliptic, acuminate and downturned at apex, acute at base, 2.5-8.5 cm long, 1-3.3 cm wide, glabrous or nearly so, often with pitlike axillary domatia below, the margins revolute; major lateral veins 4-6, obscure. Inflorescences terminal, +/- flat-topped, dichasial cymes, bearing several to many flowers, the branches minutely puberulent, each subtended by an acute bract to 1.3 mm long; pedicels ca 1.5 mm long; flowers 5-parted, ca 7 mm long (excluding pedicel); calyx 2.3 mm long, thickest at base, the lobes acute, thickened medially, imbricate, inconspicuously ciliolate, bearing minute squamellae at base inside, these shorter than nectaries; corolla pale yellow, to 8 mm wide, minutely and inconspicuously puberulent except at base, the tube 2 mm long, as broad as long, with a densely villous ring at apex inside, the lobes spreading, to ca 3.7 mm long, acute at apex, ciliolate; stamens ca 4.7 mm long, exserted at anthesis; filaments fused to tube near base, weakly connate at least below anthers; anthers connate, ca 2.3 mm long, sagittate at base; ovary pubescent; stigma 1, +/- conical, held tightly inside anthers, sticky; nectaries ovate, +/- orange, ca 0.5 mm long, pubescent near apical margin, the nectar stored in corolla tube. Follicles paired, to 15 cm long, 3-4 mm diam, terete, glabrous; seeds oblong-linear, concave on one side, ca 7 mm long, pubescent, with a tuft of trichomes at apex. Croat 14000, Foster 950. Individuals flower for at least a month, with some fruits of nearly mature size on flowering plants. Mature fruiting season unknown.