=Fam. 3. Limnadiidae.=—Body compressed; carapace in the form of a bivalve shell, the two halves capable of adduction by means of a strong transverse muscle; second antennae biramous, alike in both sexes; in the male, the first or the first and second thoracic limbs prehensile; telson laterally compressed. A. Only the first thoracic limbs prehensile in the male; the carapace spheroidal, without lines of growth; head not included within the carapace-chamber. _Limnetis_, Lovén—Compound eyes fused; anal spines absent; ambulatory limbs 10–12. _L. brachyura_, O. F. Müll (Fig. 3, p. 21).—Norway, Central Europe. B. The first and second thoracic limbs prehensile in the male; carapace distinctly bivalve, enclosing the head, with concentric lines of growth round a more or less prominent umbo. _Eulimnadia_, Packard—Carapace narrowly ovate, with few (4–5) lines of growth. _E. mauritani_, Guérin—Mauritius. _E. texana_, Packard—Texas, Kansas. _Limnadia_, Brongniart—Carapace broadly ovate, with numerous lines of growth, without distinct umbones; _L. lenticularis_, Linn.—Northern and Central Europe. _Estheria_, Rüppell—Carapace with well-marked umbones and numerous lines of growth, oval; _E. tetraceros_, Kryneki—Central Europe. _Leptestheria_,[29] G. O. Sars—Carapace compressed, oblong. Rostrum with a movable spine; thoracic limbs with accessory lappet on the exopodite. _L. siliqua_, G. O. Sars—Cape Town. _Cyclestheria_,[30] G. O. Sars. _C. hislopi_, Baird— Queensland, India, East Africa, Brazil. =Sub-Order 2. Cladocera.= The Cladocera are short-bodied Branchiopods, with not more than six pairs of thoracic limbs. The second antennae are important organs of locomotion, and are nearly always biramous; the first antennae are small, at least in the female; the second maxillae are absent in the adult. The carapace may extend backwards so as to enclose the whole post-cephalic portion of the body, or may be reduced to a small dorsal brood-pouch, leaving the body uncovered. The Cladocera or “Water-fleas” are never of great size; _Leptodora hyalina_, the largest, is only about 15 mm. long, while many Lynceidae are not more than 0·1 or 0·2 mm. in length. The =head= is bent downwards in all the Cladocera, so that parts which are morphologically anterior, such as the median eye and the first antennae, lie ventral to or even behind the compound eyes and the second antennae (_cf._ Fig. 10).