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Measurements of early ontogenetic stages of 24 species of Recent planktonic foraminifera show that they can be placed in six supergeneric groups, which in order of increasing proloculus diameter and initial whorl length are: "Globigerinoides," "Globigerina," "Globigerinita," "Globoquadrina," "Globorotalia" and "Candeina." Near surface, spinose species in the former two groups have an average proloculus diameter between 10.3 and 15.7 pm and an average initial whorl length between 35 and 63 pm. The non-spinose species which have broader (deeper) depth habitats than in the other four groups, generally possess larger proloculi and initial whorls. Within a species, individuals with large proloculi likely occur in warm, well-lit, productive waters near the surface, while those with small proloculi reflect stunted growth in deeper waters. Apparently, environmental as well as genetic factors interact to produce the wide morphological variability observed in early ontogeny. Morphometric differences in the embryonic and juvenile stages should prove to be useful for deciphering the phylogenetic relationships of planktonic foraminifera.