Suborder Valvifera
(Key references: Stimpson, 1857; Benedict, 1898; Richardson, 1905; Menzies, 1950A; Miller and Lee, 1970; Menzies and Miller, 1972; Brusca and Wallerstein, 1977, 1979A, B; Kensley and Kaufman, 1978; Brusca, 1983B, 1984; Rafi and Laubitz, 1990; Poore and Lew Ton, 1993)
Valviferans are distinguished by the unique opercular uropods that form hinged doors ("valves") covering the pleopods. Additional features that aid in recognition are the well developed coxal plates, often partly fused pleonites, absence of mandibular palps (except in the Southern Hemisphere family Holognathidae), and the penes of males arising from pleonite 1, or on the articulation of pleonite 1 and pereonite 7 (rather than on the thorax, as in all other marine isopods).
Three families and thirty-four species are represented in California waters. Mesidotea entomon, an offshore circum-Arctic species, is reported to occur as far south as Pacific Grove (California) and is the only representative of the Chaetiliidae in California. Twenty-one species in the families Arcturidae and Idoteidae occur in California's intertidal region.
(figures 73-74, 75-80, 81-86, 87-93)Key to Intertidal Species of Valvifera
| 1. | Body narrow, subcylindrical; anterior four pereopods unlike posterior three, being smaller, setose, and nonambulatory; head fused with first pereonite, leaving six free pereonites. (one species in California, I. hedgpethi) | Arcturidae: Idarcturus |
| - | Body dorsoventrally depressed; pereopods subsimilar and ambulatory; seven free pereonites. | Idoteidae 2 |
| 2. | Pleon composed of three complete pleonites and one incomplete pleonite (represented by a pair of lateral suture lines), plus pleotelson. (one species in California, C. occidentalis) | Cleantioides |
| - | Pleon with less than three complete pleonites. | 3 |
| 3. | Pleon composed of a single segment, with or without incomplete suture lines. | 4 |
| - | Pleon composed of two complete pleonites and one incomplete pleonite. | Idotea 6 |
| 4. | Antenna with multiarticulate flagellum; pleon with one pair of incomplete suture lines. | 5 |
| - | Antenna with single clavate flagellar article; pleon usually without suture lines. | Erichsonella 15 |
| 5. | Maxillipedal palp of four articles. | Colidotea 16 |
| - | Maxillipedal palp of three articles. | Synidotea 17 |
| 6. | Maxillipedal palp of four articles. | 7 |
| - | Maxillipedal palp of five articles. | 9 |
| 7. | Pleotelson posterior margin concave. | I. rufescens |
| - | Pleotelson posterior margin not concave. | 8 |
| 8. | Pleotelson posterior margin with strong median process, triangular in shape and with rounded apex. | I. ochotensis |
| - | Pleotelson posterior margin without strong median process. | I. urotoma |
| 9. | Eyes transversely (dorsoventrally) elongate; maxilliped with one, two or three coupling setae. | I. stenops |
| - | Eyes not transversely elongate; maxilliped with one coupling seta. | 10 |
| 10. | Posterior border of pleotelson strongly concave. | I. resecata |
| - | Posterior border of pleotelson not concave. | 11 |
| 11. | Pleonite 1 with acute lateral borders. | 12 |
| - | Pleonite 1 without acute lateral borders. | 13 |
| 12. | Eyes reniform; anterior margin of pereonite 1 encompassing cephalon. | I. wosnesenskii |
| - | Eyes retangular; anterior margin of pereonite 1 not encompassing cephalon. | I. schmitti |
| 13. | Pleotelson with median posterior projection. | 14 |
| - | Pleotelson without median posterior projection. | I. kirchanskii |
| 14. | Eyes circular; pleotelson median posterior projection long. | I. aculeata |
| - | Eyes with straight anterior and convex posterior border; pleotelson median posterior projection short. | I. montereyensis |
| 15. | Body not elongated (length about 3 times width). | E. pseudoculata |
| - | Body elongate (length about 7.4 times width). | E. crenulata |
| 16. | Posterior margin of pleotelson rounded; body relatively stout (length about 2.6 times width); antenna not, or barely, reaching pereonite 2; body dark purple or dark red (fading to bluish-gray in alcohol); commensal on sea urchins (Strongylocentrotus). | C. rostrata |
| - | Posterior margin of pleotelson triangular-shaped; body elongate (length about 5.5 times width); antenna reaching pereonite 3 or 4; body brown to brownish-green; not commensal on sea urchins (usually in brown algae). | C. findleyi |
| 17 | Body smooth; head without preocular horns or other projections. | S. harfordi |
| - | Body with tuberculations, carinae or bumps; head with preocular horns or other processes. | 18 |
| 18. | Pereon lacking tubercles. | S. consolidata |
| - | Pereon with tubercles. | 19 |
| 19. | Preocular horns project forward. | S. ritteri |
| - | Preocular horns project laterally. | 20 |
| 20. | Lateral borders of first four pereonites acute; each pereonite with a transverse row of three pointed tubercles. | S. pettiboneae |
| - | Lateral borders of second, third and fourth pereonites blunt; pereonites with many small tubercles. | S. berolzheimeri |
