Petrunkevitch, A. I.
A study of Palaeozoic Arachnida. Transactions of the Connecticut Academy of Arts and Sciences, 37, 69-315.
Petrunkevitch's scheme was based primarily on whether the prosoma-opisthosoma junction was broad (Latigastra), narrow (Caulogastra) or variable (Soluta) and the orientation of the chelicerae which hang down in one subclass (Stethostomata). He did not propose relationships between the orders. This scheme is significant as the fossil order Trigonotarbida makes its first appearance here, but its subclass Soluta can be criticised for being defined on what he believed to be a variable character (in fact it is restricted in all trigonotarbids).
Order ARCHITARBI (=Phalangiotarbida)
Order KUSTARACHNAE (=opilionids)
Order PHRYNCHNIDIA (=Amblypygi)