Key to Hirudisomatidae in the Western Hemisphere (adapted from that in Shelley (1996))

1. Telson distinctly narrower than caudalmost segments ................................................................................................. 2

- Telson subequal in breath to caudalmost segments. ................................................. Octoglena Wood, 1864, in part...3

2. Dorsum with broad, variegated, middorsal stripe; body vaulted in profile, sides not extending laterad; caudal metatergal margins elevated but not upturned; sternal lobes of anterior gonopods not apically hirsute, coxal lobes glabrous; Veracruz, Mexico ................................................ Mexiconium Shelley, 1996 .... M. absidatum Shelley, 1996

- Dorsum subuniformly pale yellow to white; body flattened "bell-shaped" in profile, sides extending strongly laterad; caudal metatergal margins both elevated and upturned; sternal lobes of anterior gonopods apically hirsute, coxal lobes absent; Alabama and Tennessee to North and South Carolina.............................................................................. ..................................................................................... Octoglena Wood, 1864, in part... O. gracilipes (Loomis, 1971)

3. Dorsum subuniformly pale colored, occasionally darker and somewhat orange, without horizontal bands or longitudinal stripes; Pacific Coast of North America from southwestern "mainland" of BC to southwestern Oregon; Sierra Nevada foothills of central California.......................................................................................................................... 4

- Dorsum pigmented with horizontal bands or longitudinal stripes; Pacific Coast from southwestern Oregon to Santa Cruz Co., California; Rocky Mountains of Idaho Panhandle. ...................................................................................... 6

4. Ventral lobe of anterior gonopod ultimate podomere indistinct, not overhanging dorsal counterpart, latter narrow and slightly sinuate, directed sublaterad; Placer Co., California .................................................... O. sierra Shelley, 1996

- Ventral lobe of anterior gonopod ultimate podomere distinct, clearly overhanging dorsal counterpart, latter broad, not sinuate, directed mediad; southwestern "mainland" BC to southwestern Oregon................................................. 5

5. Coxal lobe of anterior gonopod relatively short and apically sublinear, not overlying distal podomeres; southwestern "mainland" BC to Douglas Co., Oregon. .................................................................................. O. anura (Cook, 1904)

- Coxal lobe of anterior gonopod relatively long, leaning laterad, substantially overlapping distal podomeres; southwestern Oregon (Douglas, Jackson, and Josephine cos.)........................................................ O. prolata Shelley, 1996

6. Prozona with distinct, transverse bands, metazona pale yellowish to white; gonopods in situ recumbent, directed anteriad; ultimate podomere of anterior gonopod in form of flattened, terminal calyx; Idaho ....... O. claraqua, n. sp.

- Pro- and metazona with three dark, parallel, longitudinal stripes; gonopods in situ directed ventrad; ultimate podomere of anterior gonopod divided into hirsute ventral and glabrous dorsal lobes; Pacific Coast from southwestern Oregon to Santa Cruz Co., California.............................................................................. O. bivirgata Wood, 1864