1. Two small claws with broad bases and short distal ends, with or without teeth on inner margin, longest claws with pointed apices (Fig. 14 I, J); hypopharyngeal pecten (Fig. 13 K) with 6–9, usually 6 or 7, teeth; blade (Fig. 13 A) as long as segments 2 and 3 combined; lamellae of galea and chaetulae of palpifer (Fig. 14 A) broad and undivided; head capsule length 0.47–0.72 mm.................................................................................................... 2
- All claws with narrow bases, without teeth although small spines may be present on inner margins, long claws with attenuated whip-like apices (Fig. 14 K, L); hypopharyngeal pecten (Fig. 13 L, M) with 10–14, usually 11–13, teeth; usually at least as long as segments 2–4 combined; at least some lamellae of galea and chaetulae of palpifer (Fig. 14 B, C) divided at apical half to third; head length more than 0.58 mm..................................................................... 3
2. Two of the smaller posterior parapod claws sharply hooked, no claws with strong points on inner side.............................................................................................. P. (Psilotanypus) nietus Roback
- The smaller posterior parapod claws not sharply hooked, 0, 1, or 2 claws with strong points on inner side............................................................................................. P. (Psilotanypus) bellus Loew
3. Blade shorter than segments 2–4 combined (Fig. 13 B); two-segmented sensilla basiconica (SBP in Fig. 14 E) of maxillary palp shorter than bisensillum (Bs in Fig. 14 E), its basal segment shorter than its apical segment; head capsule length 0.62–0.78.................................................................................... P. (H.) freemani Sublette
- Blade at least as long as segments 2–4 combined or usually longer (Fig. 13 C–E); SBP (Fig. 14 F–H) of maxillary palp longer than Bs, its basal segment longer than its apical segment; head capsule length 0.65–1.05 mm.......................... 4
4. Blade at least as long as segments 2–4 combined, accessory blade usually nearly as long as blade (Fig. 13 C); head capsule length 0.58–0.81 mm................................................................................... 5
- Blade as long as segments 2–4 combined or, if as long as segments 2–5 combined, head capsule length over 0.92 mm; accessory blade distinctly shorter than blade (Fig. 13 D, E)........................................................................................................ 6
5. Sclerotized part of basal segment of maxillary palp (Fig. 14 F) 1.9–2.6 times as long as wide....... P. (H.) culiciformis (L.)
- Sclerotized part of basal segment of maxillary palp about 3.2 times as long as wide................ P. (H.) wilhmi Roback
6. Procercus with 18–19 setae; both mandible and basal antennal segment longer than 210 µm............ P. (H.) ruris Roback
- Procercus with 9–14 setae; mandible and basal antennal segment often shorter than 210 µm.......................... 7
7. Sclerotized part of basal segment of maxillary palp (Fig. 14 E) 45–58 µm long and 2.4–2.9, mean 2.5, times as long as wide; basal antennal segment 4.5–5.6, 5.0 times as long as wide and 3.4–4.5, mean 4.0, times as long as basal palpal segment; head capsule length 0.72–0.91 mm......................................................... P. (H.) sublettei Roback
- Sclerotized part of basal segment of maxillary palp (Fig. 14 G) 55–74 µm long and 2.7–3.5, mean 3.0, times as long as wide; basal antennal segment 5.1–6.4, mean 5.5 times as long as wide and 2.9–3.9, mean 3.4, times as long as basal palpal segment; head capsule length 0.79–1.05 mm long.............................................. P. (H.) denticulatus Sublette