=Fam. 1. Calappidae.=—Cephalothorax rounded and crab-like. The abdomen is hidden under the thorax, the antennae are small, and the legs normal in position. The afferent openings to the gill-chambers lie in front of the chelipedes. Male openings on coxae of last pair of legs. _Calappa_ (Fig. 128) circumtropical, and extending into the warmer temperate seas. _Matuta_ (Fig. 129) from the Indo-Pacific. [Illustration: FIG. 130.—Dorsal view of _Ilia nucleus_, × 1. (From an original drawing prepared for Professor Weldon.) ] =Fam. 2. Leucosiidae.=—Similar to the above, but the afferent openings to the gill-chambers lie at the bases of the third maxillipedes. Male openings on the sternum. This family contains a great number of forms, with headquarters in the tropical littoral, but extending into the temperate seas. _Ilia_ in the European seas. _I. nucleus_ (Fig. 130) common in the Mediterranean. _Ebalia_ in the Atlantic, North Sea, and Indo-Pacific. _Leucosia_ in Indo-Pacific. =Fam. 3. Dorippidae.=—Cephalothorax short and square. The abdomen is not hidden under the thorax; the antennae are large, and the last two pairs of legs are held dorsally, and have terminal hooked claws. _Dorippe_, littoral in Mediterranean and Indo-Pacific. _Cymonomus_ (Fig. 127) from deep-sea of Atlantic and Mediterranean. =Fam. 4. Raninidae.=—Similar to Dorippidae, but the cephalothorax is elongated, and the legs usually have the last two joints very broad. Several genera, chiefly in the deeper littoral zone. _Ranina dentata_ in the Indo-Pacific. =Tribe 3. Cyclometopa.= In these Crabs the carapace is circular rather than square; its frontal and lateral margins are produced into spines and there is no pointed rostrum. The mouth is square, and the third maxillipedes are greatly flattened and form a lid-like expansion over the other oral appendages. This group includes the common Shore-crab of our coasts (_Carcinus maenas_), the swimming Crabs with expanded pereiopods (_Portunus_, _Lupa_, etc.), the Edible Crab (_Cancer pagurus_), and many others.