KEY TO THE GROUPS OF PRIMITIVE ORIBATID MITES

1. The legs of the adults have 2 femora; in some or in all immature stages the legs are not monodactyle. Gnathosoma visible from above. Absence of notogaster and latero-abdominal gland.............. Palaeacaroidea

- The legs of the adults have i femur, and are monodaetyle in all immature stages. The notogaster is present ...............2

2. Notogaster divided by dorsal and superpleural "coupures". Gnathosoma visible or invisible from above. Absence of the latero-abdominal gland. One family (Protoplophoridae) is ptychoid............. Enarthronota

- Notogaster divided in a different way, or undivided........3

3. Gnathosoma visible from above. Notogaster transversely divided, although ill-defined. Presence of latero-abdominal gland. Aptychoid.............. Parhypochthonoidea

- Gnathosoma not visible from above............4

4. Ptychoid....................5

- Aptychoid...................6

5. Genital and anal openings of the adults separated by a part of the ventral plate. Nymphs scleritized and ptychoid......... Mesoplophoroidea

- Genital and anal openings of the adults not separated by a part of the ventral plate. Nymphs not scleritized............ Phthiracaroidea

6. Protero-hysterosomatic articulation......... Perlohmannoidea

- Absence of protero-hysterosomatic articulation.......... Nothroidea