Matthews, 1961
Females
Pd4 and 5 fused or separated, posterior corners of last somite of Pr symmetrical, or slightly asymmetrical. R absent, or present as single small rostral filament. Gns symmetrical, or asymmetrical. Ur of 4 somites. A1 23-24-segmented, short, not reaching the end of Pr, right one may be longer than left. Oral parts and P1-P4 agree in most details with those of male (these are described in more detail below (Fosshagen, 1978, for Mesaiokeras kaufmani). P1 Enp without outer lobe, sometimes with reduced number of setae on external margin. Distal parts of P2-P4 Enp may be covered with spinules on posterior surfaces.
Spine and seta formula: Spine and seta Mesaiokeratidae.
P5 small, symmetrical, 2-segmented. sometimes lacking on right.
Males
Last somite of Pr symmetrical, or slightly asymmetrical. R present, in form of small thin filament, or absent. Oral parts identical to that of female. A1 slightly asymmetrical, left 21-23-segmented; 8-9 and 10-12; 13-14 segments may be fused, right 20-23-segmented; 8-9, 20-21 segments may be fused; well equipped with aesthetascs on both sides. A2 Basis with 2 setae; Enp1 with 2 setae, Enp2 without distinct inner lobe; Exp 6-segmented, Exp1 with 1 seta, Exp2 with 2-3 medial seta plus 1 distal marginal seta.
Mdp Enp1 with 3 setae, Enp 2 with 9 terminal and 1 appendicular setae. Mx1 Li2 with 2 setae, Li3 with 3 setae; Exp with 8 setae. Mx2 Li1 with 5 setae, Li2-Li4 with 3 setae each. P1 Enp without outer lobe. Left P5 greatly elongated, uniramous, 5-segmented. (Mesaiokeras kaufmani, Mesaiokeras nanseni) (Fosshagen, 1978; Andronov, 1973a); right leg short, not extending beyond Exp1 of left leg.
Monotypic family containing genus Mesaiokeras Matthews, 1961.