The genus Carphalea includes shrubs and small trees that are found in sclerophyllous, dry, and xric vegetation, not a common habitat among Madagascar's Rubiaceae, and are also notable for their enlarged calyx limbs. Carphalea is characterized by its persistent stipules with 1 to several linear lobes or setae with glandular tips, its slender tubular-funnelform corollas, its fruits with several seeds, and its enlarged calyx limbs. In some species the calyx limb is symmetrically lobed, not brightly colored, and enlarges after anthesis, as the fruits develop; in other species the limb is asymmetrically lobed, bright red or red-orange, and enlarged before anthesis thus presumably functions to attract pollinators. The flowers are distylous, or sometimes tristylous in Carphalea pervilleana (Puff, 1988). Some of the species are apparently deciduous at least sometimes. Carphalea linearifolia differs from the other species in having only three small calyx lobes and one that is enlarged and petaloid; this species is poorly known, and its classification in Carphalea needs re-evaluation (Kårehed & Bremer 2007).
Carphalea has previously been circumscribed more broadly, to include plants from southeastern Africa, Socotra, and Madagascar (Puff 1988). However Kårehed & Bremer (2007) studied its systematics with molecular data and separated the African plants from those of Madagascar, into the genus Dirichletia (including Placopoda), leaving Carphalea restricted to Madagascar. Kårehed & Bremer also classified Carphalea in the tribe Knoxieae, rather than Hedyotideae as some previous authors suggested. Kårehed & Bremer found the Carphalea species they studied to be paraphyletic and grouped on two separate clades, and suggested that these might be better separated into two genera; they did not study Carphalea linearifolia. One of these clade has species with the calyx limb not brightly colored and enlarging after anthesis, including Carphalea madagascariensis, the type of the genus; the other clade has species with the calyx limb brightly colored and developed before anthesis. Kårehed & Bremer did not taxonomically separate these groups, and noted that additional study is needed to confirm the relationships they found.
Carphalea is similar to some other genera, in particular Razafimandimbisonia and Nematostylis, in the enlarged calyx lobes; a key to separating these is included on the project web page for Razafimandimbisonia. Carphalea pubescens has been reported from Madagascar in herb., but that a species of Africa now treated as Dirichletia pubescens with the calyx limb much larger than in the Madagascar species, not or hardly lobed, and very strongly ovate in outline.
The species information compiled here is based mainly on the revision presented by Puff (1988).