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Rh in the  (q.v.). Again it is clear that in parts of northern Europe, Siberia. and North America, the conditions characteristic of a glacial period are still existent; even in Scotland and Norway the last traces of glacial action are remarkably fresh, and the last remnants of great glacial centres still linger in the Alps and other lofty southern mountains. Many of the formations of this period can be shown by their fossil contents to belong to early quaternary time, but since so many of these deposits are strictly local in character, and since the fauna and flora present in any one spot have been determined by local geographical conditions which have assisted or retarded the migration of certain forms, it is a matter of extreme difficulty one may say impossibility—to reduce the Pleistocene formations to any generally applicable chronological order. For similar reasons it is impossible to define strictly the upper limit of the formations of this period, and to say where the Pleistocene ends and where the Recent or Holocene period begins.

The composition and distribution of the Pleistocene fauna and flora present many points of extreme interest. The feature of greatest importance is that man existed somewhere and in some condition before and in this period; but no really satisfactory proof has so far been forthcoming which will set back his first appearance before the beginning of the glacial period (Pithecanthropus erectus found by E. Dubois in Java is regarded as of Pliocene age). The presence of the remains of man or of his works might reasonably be taken as a criterion of the Pleistocene age of a deposit if we omit the remains of historical time. But here again it has to be borne in mind that historical time is continually being set back by archaeological research, and further, the difficulty of employing artefacts of stone as chronological indicators is shown by the fact that even at the present day implements of stone are still in use, and that different local races of early men must have been in diverse stages of development in Pleistocene as in later ages. It is, therefore, only with the utmost caution that chronological subdivisions of the period, such as those mentioned below, based upon the form and degree of finish of stone implements, can be used in anything but local correlations unless the evidence is suported by satisfactory fossils.

Next to the appearance of man the most striking characteristic of the land fauna was the existence of numerous large-bodied mammals, Elephas antiquus, for instance, attained a more excessive bulk than any other proboscidean either before or since, the woolly rhinoceros, the great hippopotamus, the cave bear, cave lion and giant deer were all larger than their living representatives. No less striking is the disappearance of these large forms together with highly specialized creatures such as Machaerodus within the same period, through the action of the same causes which had removed the bulky and specialized reptiles of an earlier geological period. The Pleistocene mammalia of Europe include Elephas antiquus, E. primogenus (mammoth), R. antiquitatus (tichorhinus) (the woolly rhinoceros), R. mercki (especially in Silesia), R. leptorhinus (south-east Europe), Elasmotherium (Silesia and south Russia), Hippopotamus major, Bos primigenius (aurochs, extinct in historical time), Bison priscus, Bison europaeus (still living in the Caucasus and Lithuania), Bos (Bubalus) pallasi (north Europe), camels in south Russia and Rumania, Equus fossilis and varieties, Cervus (Megaceros) giganteus (=hibernicus) (the great Irish " elk " and its varieties); Cervus elaphus, C. aleus, Rangier tarandus and R. groenlandicus (reindeer), Capreolus caprea, Capra ibex, Saiga tatarica, Ovibos moschatus, Felis spelaeus, Hyaena spelaea, Ursus spelaeus, badger, weasel, glutton, hare, lemming (Myodes torquatus and M. lemmus), Spermophilus, Alactaga, Arctomys, Castor fiber, Lagomys, Trogontherium. In North America there were numerous mammals common to Europe and North Asia, including the musk-ox, mammoth and horse; the mastodon held on into this period in America but not in Europe, there were also lamas, tapirs, camels (Camelus auchenia), Machaerodus, Mylodon, Procyon, Alces. In South America there was at first a very characteristic endemic fauna including Megatherium, Mylodon, Grypotherium, Lestodon, Toxodon, Typotherium, Glyptodon, Macrauchenia, Capybara, Rhea, to which were added later, Mastodon, Machaerodus, Lama and other North American forms. In Australia a very distinct assemblage of large marsupials and monotremes lived in the Pleistocene period; including Phascolus, Diprotodon, Thylacoleo, Nototherium and a large extinct Echidna; placental mammals were not then known in this region. In Madagascar the Aepiornis, Megaladapis, and certain extinct lemuroid creatures have left their remains.

The advance and retreat of glacial conditions in northern latitudes had a marked influence upon animal and plant life, and was the means of determining the present distribution of man of the living mammalia and plants; some were driven permanently southward, some northern forms still live isolated on the higher mountain regions, others like the reindeer and musk-ox returned northward as soon as the conditions permitted. The apparently curious admixture of what are now often regarded as tropical or sub-tropical forms (lion, hyena, rhinoceros and elephants) with cold-temperate or arctic genera, presents no real difficulty, since their distribution was doubtless merely a matter of food supply; and some of these, like the woolly rhinoceros and mammoth, were provided with a thick hairy pelt.

Although in the main the arrangement of land and sea was little different from that which obtains at the present time, one or two features existed in the Pleistocene period which had a considerable influence on faunal migration. For instance, the absence of the Bering Straits permitted free communication between Europe and North America, and the absence of the Straits of Dover allowed a similar interchange between Great Britain and France; while an extension of the sea in the Caspian region and of the Arctic Sea in northern Russia acted as a bar to free passage between Europe and Asia in those regions.

The formations of Pleistocene age, other than those of direct glacial origin, include deposits on time floors of caves in limestone and dolomitic rocks, calcareous sinter (travertine or tufa) formed by springs, ancient river and lake alluvial and lacustrine terraces, elevated marine beaches, submerged forests, ancient lake deposits and peat beds, laterite, loess and sand dunes.

Some of the prevalent styles of classifying the deposits of the glacial formations of this period are mentioned in the article. The following subdivisions are often employed by European geologists: a younger division, Reindeer time = Magdalénien stage; a middle division, Mammoth time = Salutréen stage; and an older division, Elephas antiquus time = Chelléen stage. While some authors include all the above in the " glacial period," others would place the Magdalénien in a post-glacial division. The terms Magdalénien, &c., are really archaeological, based upon the characters of the implements found, in the deposits, and like the similar terms " eolithic " and " paleolithic " they are of little value in geological chronology unless they are supported by palaeontological evidence.

See E. Geinitz, Das Quartar von nord Europa (Stuttgart, 1904), with very full references; T. C. Chamberlin and R. D. Salisbury, Geology, vol. iii. (New York, 1906), for references to American authorities.

PLEONASM (Gr., from , to abound or be superfluous, , comparative of , many, great, large), redundancy or superfluity in speaking or writing, hence an unnecessary work or phrase. The word, more usually in the Latin form "pleonasmus," is used in pathology of an abnormal growth or formation.

 PLESIOSAURUS, an extinct marine reptile belonging to the Order Sauropterygia, which characterized the Mesozoic period and had an almost world-wide distribution (see ). The animal is best known by nearly complete skeletons from the Lias of England and Germany. It was named Plesiosaurus (Gr. more-lizard) by W. D. Conybeare in 1821, to indicate that it was much more nearly a normal reptile than the strange

(From a memoir by Professor W. Dames in the Abhandlungen der kg. preuss. Akad. d. Wiss.)

Plesiosaurus guilelmi-imperatoris, restored.

Ichthyosaurus, which had been found in the same Liassic formation a few years previously. It has a small head, a long and slender neck, a round body, a very short tail, and two pairs of large, elongated paddles. The snout is short, but the gape of the mouth is wide, and the jaws are provided with a series of conical teeth in sockets, much like those of the living gavial