_a._ The four-jointed ramus of the second antenna with four swimming hairs. _Macrothrix_, Baird: the first antennae of the female flattened, curved. The intestine simple, straight. _Streblocerus_, Sars: first antennae of the female very little flattened, curved backwards and outwards. The intestine coiled, the stomach with two forwardly-directed caeca. _b._ The four-jointed ramus of the second antenna with only three swimming hairs. _Drepanothrix_, Sars. iii. Six pairs of thoracic limbs; the labrum provided with an appendage. _Acantholeberis_, Lilljeborg: appendage of labrum long, pointed, and setose. Intestine without caecum. _Ilyocryptus_, Sars: appendage of the labrum short, truncated. Intestine with a caecum. =B.= Both rami of second antenna three-jointed. =Fam. 6. Lynceidae=[35]: five or six equidistant pairs of thoracic feet. Intestine coiled. i. Six pairs of thoracic limbs. Head and thorax separated by a deep depression. Intestine with one caecum, stomach with two. Female carries many summer-eggs. _Eurycercus_, Baird. ii. Five pairs of thoracic limbs. Head and thorax separated by a slight groove or not at all. Anterior digestive caeca absent. Female carries only one or two summer-eggs. A. Body elongate, oval. _a._ Head carinate, the eye far from the anterior cephalic margin. _Camptocercus_, Baird: body laterally compressed. Second antennae with seven swimming hairs. Telson more than half as long as the shell. _Acroperus_, Baird (Fig. 23): body compressed. Second antennae with eight swimming hairs, of which one is very small. Telson less than half as long as the shell. _b._ Head not carinate, the eye near the anterior cephalic margin. _Alonopsis_, Sars: terminal claws of telson with _three_ accessory teeth. _Alona_, Baird: terminal claws of telson with one accessory tooth (includes sub-genera _Leydigia_, _Alona_, _Harporhynchus_, _Graptoleberis_). _Peracantha_, Baird (Fig. 14): terminal claws of telson with two accessory teeth (includes sub-genera _Alonella_, _Pleuroxus_, _Peracantha_). B. Body small, spheroidal; the head depressed. _Chydorus_, Leach: compound eye present. _Monopsilus_, Sars: compound eye absent. Tribe II. GYMNOMERA, Sars.—The carapace forms a closed brood-pouch, which does not cover the body; all the thoracic limbs prehensile. =Fam. 7. Polyphemidae=: four pairs of thoracic limbs, provided with a gnathobase.