The study of the mandible of the Bathynellacea yielded the following results: The family Leptobathynellidae Noodt, which is according to Schminke (1973) synonymous with the family Parabathynellidae, is a valid taxon. This taxon is characterized by a corpus mandibulae being longer than high, a pars incisiva being localized in the antero-distal region of the corpus mandibulae, a particular arrangement of the molar claws on the corpus mandibulae, parallel to the longitudinal axis of the mandible, a direct implantation of the adductors on the corpus mandibulae, and a parallel orientation of the general developmental axis and the connective border. These features prove the strong differentiation of the mandible that characterize the present family against that of the Parabathynellidae, in which the corpus mandibulae is as long as high, the pars incisiva is individualized over the entire distal length of the corpus mandibulae, the molar claws are located on a lobe (molar plate) and are arranged parallel to the transverse axis of the mandible, the 2nd adductors are implanted on the labro-mandibular apodeme, and the general axis of development is inclined in relation to the connective border. A number of features connect the Leptobathynellidae and the Parabathynellidae, viz. the inclination, in antero-posterior sense, of the connection level of the mandibles on the cephalic capsule, the one-segmented mandibular palp, the reduced size and non-prehensile nature of the molar part comprising the claws, and the total absence of transverse mandibular muscles. These characters separate them clearly from the Bathynellidae, which show a postero-anterior inclination of the connecting level of the mandibles on the cephalic capsule, a 3-, 2-, or 1-segmented mandibular palp of large size and prehensile, a molar part provided with teeth, and the presence of transverse adductor muscles 5a. Given the structural relations and the degree of differentiation of the mandibles characteristic of each family, we propose a division of the order Bathynellacea Chappuis into two great evolutionary assemblages, the suborder Bathynellidea (comprising the family Bathynellidae Grobben) and the suborder Parabathynellidea (uniting the families Parabathynellidae Noodt and Leptobathynellidae Noodt).