This species (i.e., the type variety) is characterized by its narrowly elliptic leaves that are acute at the apex; bilobed stipules; pyramidal inflorescences with well developed peduncles and primary axis, which is branched to 3-4 orders with 2-3 pairs of secondary axes that are shorter than the primary axis; rather short calyx limbs; rather small corollas; and subglobose or didymous fruits.

This species was described by Bremekamp with three varieties, which were distinguished by details of the pubescence: Apomuria angustifolia var. angustifolia was separated as having glabrous stems, adaxial petiole surfaces, stipules, leaves and inflorescence axes; Apomuria angustifolia var. pubirama was separated based on its pubescent stems, adaxial petiole surfaces, stipules, leaves, and inflorescence axes; and Apomuria angustifolia var. pubescens was separated based on its pubescence similar to that of var. pubirama and also distributed on the lower leaf surface. No additional description was provided, and the type of var. pubirama has not been seen but the type of var. pubescens does not seem to represent the same species as var. angustifolia. In particular these differ in leaf size, somewhat smaller and broader in var. pubescens, and in inflorescence arrangement, with the inflorescences of var. pubescens corymbiform-rounded to flat-topped due to the very short primary axis and only one pair of secondary axes that are longer than the primary axis. Apomuria angustifolia var. pubescens is generally more similar to Apomuria perrieri and other such species. Bremekamp provided detailed morphological information only for the type variety, so until a specimen is seen it is not possible to determine if var. pubirama is similar to var. angustifolia or var. pubescens. The habitat of var. angustifolia was not detailed, but the other two varieties are documented from tsingy substrates. Bremekamp separated several Apomuria species based mainly on leaf characters, and these will need re-evaluating when more material is available. The plants described as var. pubescens seem to be deciduous, and the others may be as well. When Apomuria angustifolia was transferred to Psychotria, the taxonomic status and nomenclatural change of the varieties was not addressed, so these three varieties only have names in Apomuria; for more detailed information about these individual varieities, see their individual web pages.