1. Body laterally flattened. . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Body subcylindrical, or pereopod II bearing long sital fringes longer than the perepod segments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corophiidae, Apocorophium lacustra
2. First antenna lacking an accessory flagellum, not subterranean. Perepod I and II gnathopods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyalellidae, Hyalella sp.
First antenna bearing an accessory flagellum. . . . . . . . . . . . . . 3
3. First antenna assessory flagellum consisting of one distinct flagellomere and one reduced termiinal flagellomere, or just one conical flagellomer; eyes present or absent. Crangonycitidae, 4
First antenna accessory flagellum consisting of two to seven distinct flagellomeres; eyes present. . . . . . . . Grammaridae, 5
4. Uropod III biramous, exopod longer than peduncle, endopod reduced, scale-like. . . . . . . . . . . .. Crangonyx setodactylus
Uropod III uniramous, endopod shorter than peduncle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synurella dentata
5. Uropod III with endopod 0.20 times the length or less of exopod. . . . . . . . . . . . . . . . . . . Echinogammarus ischmus
Uropod III with endopod 0.40 times the length or more of exopod. . . . . . . . . . . . . . . . . . . . . . . . . . . Gammarus, 6
6. Antenna 2 richly setose; pereopod 7, distoposterior margin of basis not forming a free lobe but fusing directly (or nearly so) to segment proper, junction moarked by a cluster of loong setae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gammarus fasciatus
Antenna 2 not richly setose; pereopod 7, distoposterior margin forming a free lobe, junction not marked by a cluster of long setae (although a few sort ones may be present). . . . . . . . . 7
7. Posterior margin of basis of oereopod 7 with long setae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gammarus pseudolimnaeus
Posterior margin of basis of pereopod 7 with short setae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gammarus minus