The difference in size variations of different populations of Diacria trispinosa, Clio pyramidata, D. quadridentata, Cuvierina columnella, Cavolinia uncinata, and C. longirostris is described. The latitudinal variation in size is demonstrated. The influence of juvenile, minute, skinny, and adult stages on the size variation in a population is discussed. “Hyalaea limbata”, described by D’Orbigny in 1836, is re-described as Cavolinia longirostris forma limbata. In Cuvierina columnella the forma atlantica is described as new. The formae limbata and atlantica are the largest forms in the respective species. Growth in length of Cuvierina columnella and growth in thickness in Cavolinia gibbosa and C. globulosa are also discussed. Size frequency distribution in populations of Cavolinia globulosa, Clio pyramidata, Diacria trispinosa, D. quadridentata, Cavolinia longirostris, C. uncinata, C. tridentata, Cuvierina columnella, Cavolinia inflexa, Creseis virgula, and Styliola subula, and the influence which is responsible for the skewing to the right or to the left of the various curves are explained. The phenomenon of shell metamorphosis and the growth type of the juveniles are also discussed.