Key to Genera of Amphithoidae
Based on Stebbing (1906) and Barnard (1940).

As will be seen from this key and from comparison with the description and figures which follow, the specimens are distinguished as Pleonexes by their possession of a mandibular palp, lack of an accessory flagellum, the apical widening of the 6th segment of peraeopods 3–5. and the presence of hooks on the telson. They are further separated from Exampithoë, in that the latter has the dactylos of peraeopod 3 reverted. They appear to come closest to Ampithoe from which they are distinguished by the apical broadening of the propod of peraeopods 3–5.

The only recent paper dealing with the genus Pleonexes is one by Miloslawskaja (1939) which I have been unable to obtain. It deals with P. gammaroides. Chevreux and Fage (1925) list two species of Pleonexes, P. gammaroides and P. ferox The Island Bay specimens appear to belong to neither of these but come closer to P. gammaroides.

The distribution of these two species, as also of the three doubtful species which Stebbing (1906) lists, is confined to the northern hemisphere, with the exception of P. dubius from Mauritius, in the Indian Ocean, and this Stebbing (1888) considers more likely a species of Sunamphitoë Barnard has recorded four genera from South Africa, Cymadusa, Macropisthopous, Ampithoë and Exampithoë. Sunampithoë has been recorded from New Guinea (Ruffo, 1948). Sheard (1937) lists Ampithoe and Grubia species from Australia, most of which Pirlot (1939) places in the synonymy of Cymadusa filosa. But none of these genera can be made to include the Island Bay specimens. Probably the discontinuity of Pleonexes is due to lack of collecting in the intervening areas.

It will be noticed that there are variations in the spelling of the root form of the family name and generic names formed from it. It is regrettable that all three variants must be accepted, unless one is to disregard the rules of nomenclature.