ENTOMOSTRACA.
Char.—Body with more or fewer segments than fourteen ; integument chitinous, forming in some a bivalve shell, eyes sessile.
Small bivalve entomostracous Crustacea are found in all strata, and attain their maximum size in the older rocks. Minute Ostracoda, related to the recent Cyp) is (fig. 10, 5), swarm in the laminated fresh-water clays of the Wealden ; whilst the marine Cytheridce assist with their multitudinous atoms in building up the chalk. Amongst the Phyllopods, the gregarious Estheria covers the slabs of Wealden and of Keuper with crowds of bivalve shells which have been commonly mis taken for Cyclades and Posidonomyce. Estheria abounds in the Paleozoic Entomoetraca.
r. Leperditia Baltica, Wahl. ; U. Silurian, Gothland.
z. Entomoconchus Scouleri, WC.; Carboniferous, Ireland.
3. Beyrichia complicate, Salter; L. Silurian, Wales.
4. Dithyrocaris Scouleri, WC.; Carboniferous, Ireland.
5. Pterygotus Anglicus, Ag. ; Old Red Sandstone, Ludlow.
6. Bellinurus bellulus, Konig.; Carboniferous, Coalbrookdale.
7. Menus Davisii, Salter ; L. Silurian, Bala.
S. Phacops caudatus, Brun.; U. Silurian, Dudley.
9. Calymene Blumenbachii, Br.; U Silurian, Dudley. to. Trinucleus ornatus, Sternb.; L. Silurian, Britain. 1. Agnostus trinodus, Salter ; L. Silurian, Britain.
Caithness flags of the middle Devonian series. The globose Entomoconchus (fig. 9, 2) is found in the carboniferous lime stone ; Leperditia (fig. 9, r) in the Silurian rocks of the north ; and Beyrichia (fig. 9, 3), which is characteristically Silurian, may be distinguished from the young forms of Trilobites by the unsymmetrical shape of its separated valves. Other pa laeozoic (Ceratiocaris and Hymenocaris) related to the recent Hebalia, and having a conspicuous tail, occur in the upper and lower Silurian strata ; the genus Leptocheles (M‘C.) was founded on the tail-spines of these Crustacea. Dithyro
caris (fig. 9, 4), which resembles the recent Apia 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 Benin vrus (fig. 9, 6), a small Pcecilopod, differing from the recent king-crab (Limu/us) in the moveable condition of the body segments. But the most extraordinary of the palozoic Crus tacea are the Eurypterus, Himantopterus, and Pterygotus (fig. 9, from the Upper Silurian and Old Red 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 Linndi ; or as the representatives of the larval condition of the stalk-eyed Malacostraca. But the following structures show 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 bibbed tail. Eurypterus has eight feet ; the others have three pairs of limbs—viz., the chelate antenna, 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 problematieus 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, cal led " Protichnites," subsequently to be described.