One damaged specimen remains in the Lamarck Collection, Geneva, from the original type series of <i>Rotalia trochidiformis</i> (Lamarck) the remainder of which was included in the Defrance Collection in Caen, all of which was destroyed in World War II. Restudy of this sole remaining paralectotype, together with topotypes and other material held in the British Museum (Natural History) has revealed the morphological changes that occur with ontogeny in this species and has made an emended generic diagnosis necessary. In the adult, <i>R. trochidiformis</i> develops open septal canals with a tributary system of secondary and tertiary fissures by resorption and a dense mass of vertical pillars which effectively destroy the umbilical cover plate seen in the juvenile. The septal canals connect with the umbilical canal via supplementary apertures and with vertical canals between the umbilical pillars. They also connect with the foraminal passage via subsutural canals. The idea the <i>Rotalia</i> should be reassigned to the Discorbidae is rejected and the integrity of the Rotaliidae is affirmed.