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.