CRUSTACEA. 523 



The Order Amphipoda consists of animals that live in water, burrow in sand, 

 or become parasitic on fishes. The abdomen is well developed, and bears limbs for 

 leaping or swimming. They always swim on their sides. The common sand- 

 hopper on the shore of the sea belongs to this order. The Order Anisopoda has a 

 long body, convex above and flattened below, and has affinities with the Isopoda, of 

 which the common wood-louse is an example. 



The Subclass Podopthalmata has compound eyes at the extremity of a pair of 

 movable stalks; the head and thorax are generally united, covered by a single 

 piece of shell, and called the cephalothorax ; this includes the antennse, eyes, mouth, 

 jaws, feet, etc. The remaining segments form an abdomen, which frequently ter- 

 minates in a caudal fin. Common examples are the squill and the small edible crab. 

 There are several orders in this subclass, only three of which are Palaeozoic, viz.: 

 Phyllocarida, Decapoda, Tetradecapoda. 



The Phyllocarida has cephalic, thoracic, and abdominal segments. The cara- 

 pace has no regular hinge. The living representative is Nebalia, which inhabits the 

 sea at moderate depths. The Decapoda are stalk-eyed, and the head and thoracic 

 segments are united in a cephalothorax, incased in a common shell, and have the 

 branchial organs inclosed on the sides of the cephalothorax. The true thoracic legs 

 are almost always ten, whence the name of the order. The Tetradecapoda have 

 their relations with the Decapoda. 



SUBCLASS AND ORDER CIRRIPEDIA. 



Family Balances. — Palaeocrusia, Protobalanus. 

 Family Lepadid^e. — Lepidocoleus, Strobilepis, Turrilepas. 



SUBCLASS ENTOMOSTRACA. 



Order Ostracoda. 



Family Beyrichiidje. — Beyrichia, Beyrichona, Hipponicharion, Primitia. 



Family Cyprid.e. — Candona. 



Family Cytherid2B. — Cytherella, Cytheropsis. 



Family Faberiid^e. — Faberia. 



Family Leperditiid^e. — Aparchites, Isochilina, Leperditia. 



Order Phyllopoda. 



Family Estherhd^e. — Estheria, Leaia, Schizodiscus. 

 Family Uncertain. — Lepidilla, Lepiditta. 



Order Trilobita. 



Family Acidaspidje. — Acidaspis. 



Family Aglaspid^e. — Aglaspis. 



Family Agnostid^e. — Agnostus, Microdiscus, Shumardia. 



Family Asaphid^e. — Asaphus, Barrandia, Megalaspis, Nileus, Ogygia, Sym- 



physurus. 

 Family Bathyurid^e. — Asaphiscus, Bathyurellus, Bathyuriscus, Bathyurus. 

 Family Bronteid^e. — Bronteus. 

 Family Calymenid^e. — Calymene, Homalonotus. 



