ENTOMOSTKACA. 47 



may be distinguished from the young forms of Trilobites by 

 the unsymmetrical shape of its separated valves. Other pa- 

 laeozoic Phyllopods (Ceratiocaris and Hymenocaris) related to 

 the recent Nebalia, and having a conspicuous tail, occur in the 

 upper and lower Silurian strata ; the genus Leptocheles (M f C.) 

 was founded on the tail-spines of these Crustacea. Dithyro- 

 caris (fig. 9, 4), which resembles the recent Apus in the hori- 

 zontal compression of its carapace, is found in the carboni- 

 ferous limestone. The lower coal measures also contain, in 

 their nodules of clay-ironstone, frequent examples of Bellin- 

 urus (fig. 9, 6), a small Pcecilopod, differing from the recent 

 king-crab (Limulus) in the movable condition of the body- 

 segments. But the most extraordinary of the palaeozoic Crus- 

 tacea are the Eurypterus, Himantopterus, and Pierygotus (fig. 

 9, 5), from the Upper Silurian and Old Eed Sandstone, of 

 which some far surpassed the largest living lobster or king- 

 crab in size. They have been considered an extinct family, 

 related to the Limuli ; or as the representatives of the larval 

 condition of the stalk-eyed Malacostraca ; but the following 

 structures shew an affinity to the Ostracoda. Their carapace is 

 comparatively small, with compound eyes on the antero-lateral 

 margins ; the body segments are eleven or twelve in number, 

 without appendages, and terminated by a pointed or bilobed 

 tail. Eurypterus has eight feet ; the others have three pairs 

 of limbs — viz., the chelate antennae, the foot-jaws, and the na- 

 tatory feet, with their fin-like palettes, which spring from the 

 under side of their cephalo-thorax. The surface of the body 

 and limbs often presents a peculiar imbricated sculpture, which 

 caused them at one time to be regarded as fishes by Agassiz. 

 The Pterygotus problematicus is supposed to have attained a 

 length of seven feet, and some of the others were a yard long. 

 Crustacea of this magnitude may have formed tracks on the 

 sea-bed, like those on the Potsdam sandstone of America, 

 called " Protichnites " (fig. 82), subsequently to be described. 



