This genus has an interesting disrupted distribution, one species, H. jenmani K. Schum. known from the Mazaruni River of British Guiana, all the remaining species being known from the Rio Negro, Rio Guainia, and portions of the Upper Rio Orinoco, and various tributaries of these rivers (Rio Curicuriary and Rio Içana in Brazil, Rio Vaupes in Colombia, and Rio Atabapo, Rio Temi, and Rio Pacimoni in Venezuela). In the intervening large region no collections of the genus are known.
This geographical hiatus is interesting in that the British Guiana species differs in various respects from all the other species of the genus, while the species from Brazil and southwestern Venezuela are a closely knit group with detailed relationships. The species from British Guiana is the only one of the genus in which all the corolla-lobes show pubescence on their inner surface, whereas in all the other species only one of the corolla-lobes is pubescent within. In the species from British Guiana the calyx-lobes are not cut down all the way, thus leaving a partial tube or hypanthium, whereas in all the other species, the calyxlobes are deeply cut to the base with overlapping or imbricating margins; they are also more foliaceous and conspicuous than in H. jenmani. Finally, it should be noted that in H. jenmani the inner surface of the stipules has loose fairly evident ascending hairs, whereas in the other species the pubescence consists of a tomentum of very short and minute puberulence. These differences may be expressed in the following key to the species.