Type species. Ilyodromus stanleyanus (King, 1855)
Diagnosis (modified after Shearn et al. 2014). Carapace of females larger than males but otherwise with similar shape and form, often with longitudinal striations of varying complexity, in lateral view sub-rectangular and usually straight along dorsal hinge with LV slightly overlapping RV, in dorsal view often elongated and compressed. Internal view of both valves with CIL usually broad anteriorly and posteriorly, with or without inner lists in the LV, and most species without prominent selvage in the RV, CIL of LV with anteroventral peg (remnants of an inner list) in most species and in some species with a second peg posteroventrally, these pegs of varying size and structure. A1 second segment with two-segmented sensory RO having distal flagella of varying lengths, sometimes appearing as third segment under low magnification; this segment with a long dorso-distal seta. A2 with natatory setae highly variable in length. Md palp with α seta short and spine-like distally, but with broad base; β seta stout, covered with long setules, and almost of the same length as the α seta; γ seta elongate and with short setules covering the distal half. Mx with seven setae on first palp segment and a spatulate second palp segment with three claws and three setae. Claw-like Sp of the CR hirsute and jointed at the base in all species, but more thin and bristle-like morphologies also possible. Male Mx prehensile palps with hook shaped distal segments. Hemipenes with lobes ls and ms highly variable in shape.
Remarks. This genus is placed alongside two other genera (Candonocypris Sars, 1896 and Herpetocypris Brady and Norman, 1889) within the tribe Herpetocypridini Kaufmann, 1900. Ilyodromus can be distinguished from Herpetocypris by the often claw-like Sp on the CR, the reduced basal branch on the CR attachment (triangular in Herpetocypris), and the absence of an additional branch on the CR attachment (present in Herpetocypris). It can be differentiated from Candonocypris by having only a single lateral seta on the penultimate segment of the seventh limb (two lateral setae in Candonocypris).