Peridiscaceae is a family of flowering plants in the order Saxifragales.[2] Four genera comprise this family: Medusandra, Soyauxia, Peridiscus, and Whittonia.,[3] with a total of 12 known species.[4] It has a disjunct distribution, with Peridiscus occurring in Venezuela and northern Brazil, Whittonia in Guyana,[5] Medusandra in Cameroon, and Soyauxia in tropical West Africa.[6] Whittonia is possibly extinct, being known from only one specimen collected below Kaieteur Falls in Guyana. In 2006, archeologists attempted to rediscover it, however, it proved unsuccessful.[3]
The largest genus is Soyauxia, with about seven species. Medusandra has two species. Peridiscus and Whittonia each contain one species. The Peridiscaceae are small trees or erect shrubs of wet tropical forests.
It was not until 2009 that all four of the genera were united into a single family.[3] Peridiscus and Whittonia are clearly close relatives. This pair, and the other two genera have long been considered anomalous, being variously classified by different authors.
The following description was created by combining descriptions of Medusandra and Peridiscus by John Hutchinson[7] with descriptions of Soyauxia, Peridiscus, and Whittonia by Clemens Bayer.[5]
Peridiscaceae are small trees or erect shrubs. The leaves are stipulate, alternate, and simple, with margins that are entire or remotely crenulate (Medusandra). The petiole is pulvinate, at its apex, sometimes obscurely so. The stipules are in the axils of the leaves, sometimes enclosing an axillary bud.
The inflorescence is a cluster of axillary racemes or spikes, the clusters often being reduced to a pair of racemes or to a single raceme. The flowers are bisexual and actinomorphic. The sepals are 4 to 7 in number, and free, that is, separate from each other. Medusandra and Soyauxia have five petals. Peridiscus and Whittonia have none.