Portnallia bognorensis was a very small early Eocene mastixioid plant known from its compact multi-chambered fruit stones. Several seeds were protected within curved cavities separated by narrow fibrous partitions, giving the fruit a complex internal structure despite its tiny size.

The fruit is especially useful for showing how much mastixioid diversity could be packed into the London Clay flora. It resembles Lanfrancia internally but is markedly smaller. Material once separated as Portnallia sheppeyensis is included within the modern concept of this species, broadening the size range represented by the accepted taxon.