PLE 



PLI 



It skirts the London clay within the 

 London chalk basin, and appears also in 

 the Isle of Wight. 



PLATEAU. A terra often exclusively 

 applied to an elevated plain ; it is, how- 

 ever, sometimes used to denote a great 

 extent of country considerably raised 

 above the rest of the land, and having 

 its mountains, plains, and valleys ; as in 

 the minor plateau of Albania, and the 

 great plateau of Central Asia. See 

 Table Land. 



PLATFORM. When a mountain ap- 

 pears as if its summit were cut off, 

 leaving a level and horizontal surface, it 

 forms a platform, or is said to be trun- 

 cated. See Mountain. 



PLA'TINUM {plata, silver). A white 

 metal found in the auriferous sand of 

 certain rivers in America. Reduced to 

 a state of extreme division, it has the 

 appearance of sponge ; and it is then 

 called spongy platinum, and is used as a 

 pyrophorus. 



PLECTO'GNATHI (TrXeKTo?, twisted, 

 7va0of, a jaw). An order of Fishes, in 

 which the bones of the upper jaw are 

 united to each other and to the head. 



PLEI'OCENE (TrXe/oji/, more, Kaivb^, 

 recent). By the terms Older and Newer 

 Pleiocene, are denoted two divisions of 

 the Tertiary Period which are the most 

 modern, and of which the largest part of 

 the fossil shells are of recent species. 

 The newer formation is also termed 

 pleistocene. 



PLEIOSAU'RUS (TrAetW, more, araXipa, 

 a lizard). A species of animal, the re- 

 mains of which are found in some of the 

 clay beds of the oolites ; it seems to have 

 been intermediate between the plesid- 

 saurus and the ichthyosaurus ; the teeth, 

 vertebrae, bones of the extremities, &c., 

 being more like the corresponding parts 

 of the former, but the animal resembles 

 the latter in the absence of apparent 

 neck ; and from this greater analogy 

 with reptiles it has received its name. 



PLEI'STOCENE ( TrXelo-roc, most, 

 Kaii/or, recent). The newest of the ter- 

 tiary strata, which contain the largest 

 proportion of living species of shells. 



PLE'ONASTE. Ceylonite. An alu- 

 minate of protoxide of iron and mag- 

 nesia. 



PLESIOSAU'RUS (7rXrj<rt'oi/, near to, 

 (Tovpa, a lizard). An extinct genus of 

 Saurian reptiles, which united to the 

 head of a Lizard the teeth of a Croco- 

 dile; a neck of enormous length, re- 

 sembling the body of a serpent ; a trunk 

 263 



and tail having the proportions of an 

 ordinary quadruped, the ribs of a Chame- 

 leon, and the paddles of a Whale. They 

 appear to have lived in shallow seas 

 and estuaries, and to have breathed air 

 like the Ichthyosauri and our modern 

 Cetacea. 



PLEURE'NCHYMA (7rXei;pa, the 

 side, e^x^Ma* infusion). A designation of 

 the woody tissue of plants, consisting of 

 very slender, tough, transparent, mem- 

 branous tubes, tapering acutely to each 

 end, lying in bundles, and communi- 

 cating with one another by invisible 

 pores. 



PLEURONE'CTIDiE { pleuronectes, 

 the flounder; from nXevpa, the side, 

 vT)Krrj?, a swimmer). The Flat-fish or 

 Flounder tribe ; a family of Malacopte- 

 rygious or soft spined fishes, characterized 

 by extreme flattening of the body, and 

 by deficiency of symmetry. The term 

 pleuronectes, or side-finned, is calculated 

 to produce an erroneous ; opinion what 

 are usually called the belly and back of 

 these fishes being actually the sides, 

 though diflTering remarkably in colour. 



PLEURO'PTERA (TrXeupa, the side, 

 TTTepov, a wing). A tribe of quadrupeds, 

 generally known as Flying Lemurs, 

 Flying Cats, and Flying Foxes. They 

 are generally arranged under the order 

 Carnassiers, and are placed by some 

 zoologists in the division Cheiroptera. 



PLEUROTOMA'RIiE. A sub-family 

 of the TrochidcB, named from the pleu- 

 rotomaria, a fossil trochiform shell, 

 having a slit in the outer lip. 



PLEUROTO'MIN^. The slit-shells ; 

 a sub-family of the Stromhidce, or Wing- 

 shells, named from the typical genus 

 pleurotoma, and characterized by a deep 

 lobe, or slit, at the top of the outer lip ; 

 the spire, in general, is very long and 

 the shells themselves often spindle- 

 shaped. 



PLI'CIPENNES (^plica, a fold, penna, 

 a wing). The third section, according to 

 Latreille, of the Neuropterous insects. 

 They constitute the genus phryganea, 

 and are known by the name of caddis- 

 worm. They form the order Trichoptera 

 in the system of Kirby and Spence. 



PLINLIMMON ROCKS. A subordi- 

 nate group of the Cambrian series, con- 

 sisting of the greywacke of various 

 qualities ; it corresponds with the Grey- 

 wacke Range of the Lammermuir, ex- 

 tending from St. Abb's Head, on the east 

 coast of Scotland, to the Mull of Gallo- 

 way. 



