P L E 



[ 358 ] 



P L E 



there are two, that with the greatest 

 number of points may be Plectro- 

 dus pleiopristris. Murchison's 



PLEISTO'CENE. A sub-division of the 

 Pleiocene division, implying that 

 it contains the maximum proportion 

 of the recent organisms, being from 

 90 to 95 per cent, of existing forms. 

 The Pleistocene deposits may be 

 defined as those in which more 

 then three-fourths of the fossils are 

 of existing species. 



PLEROSA'URTJS. A fossil saurian of 

 the lias and oolite. 



PLESIOSAT/RUS. (from TrK^alo^, near 

 to, and ffavpa, a lizard, Gr.) We 

 are indebted to the researches of the 

 Rev. "W. Conybeare for our earliest 

 acquaintance with the plesiosaurus. 

 A genus of extinct amphibious 

 animals, nearly allied to the Ich- 

 thyosaurus. Cuvier says this in- 

 habitant of the ancient world, is 

 perhaps the most heteroclite, and 

 appears to merit the name of mon- 

 ster above all others. " Get habit- 

 ant de 1'ancien monde est peut- 

 etre la plus heteroclite et celui de 

 tous qui paroit le plus meriter le 

 nom de monstre." It united the 

 teeth of a crocodile to the head of a 

 lizard; its neck was of enormous 

 length, exceeding that of its body, 

 and resembling the body of a ser- 

 pent ; it possessed a trunk and tail 

 of the proportions of an ordinary 

 quadruped ; to all these were added 

 the ribs of a cameleon, and the pad- 

 dles of a whale. The teeth were 

 conical, very slender, curved 

 inwards, finely striated on the 

 enamelled surface, and hollow 



, throughout the interior. Five or 

 six species of the plesiosauri are 

 known ; they appear to have lived 

 in shallow seas and estuaries, and, 

 in the opinion of some, they swam 

 upon or near the surface, having 

 the neck arched, like the swan, and 

 darting it down at the prey within 

 reach. Prodigious numbers of re- 



mains are found in the lias. Ver- 

 tebra and teeth are found in the 

 Hastings beds. Some of the plesi- 

 osauri were upwards of twenty feet 

 long. The plesiosaurus is supposed 

 to have been oviparous. Of all the 

 species yet discovered the most 

 extraordinary is the plesiosaurus 

 dolichodeirus, or long-necked plesi- 

 osaurus : the neck of this animal is 

 equal to half the entire length of 

 the body and tail united, and is 

 composed of 35 vertebrae ; the back 

 of 27, and the tail of 28 ; altogether 

 90 vertebras. The head does not 

 exceed one-fifth of the neck in 

 length. Geological Transactions. 



PLE'TA. An orthocerite limestone, of 

 a dull red and dingy-grey ; an 

 earthy and slightly consolidated 

 limestone ; with thin seams of clay. 

 It is placed amongst the lower 

 silurian rocks of Russia. 



PLEFROCY'STI. The third class of 

 echini. 



PLETJRO'TOMA. A genus of spiral 

 univalves, which have a narrow 

 aperture and long channel: the 

 outer lip is distinguished by a 

 transverse slit, about the middle or 

 upper part. Many species have 

 been found fossil in the tertiary 

 deposits, more especially in the 

 Paris basin, and in the London 

 clay. 



PLEUROTOMA'RIA. A fossil genus of 

 turbinated, spiral, univalve shells 

 belonging to the family Turbinacea. 

 They are found only fossil, and 

 ccur in the inferior oolite. 



PLI'CATED. (from plica, Lat. to fold.) 

 Plaited; folded. 



PLI'OCENE. | (from TrXetW, more, and 



PLBI'OCENE. ) KCHVOS, recent, Gr.) 

 The name given by Sir C. Lyell to 

 a division of the supracretaceous 

 group, or tertiary strata. The 

 tertiary series Sir C. Lyell divided 

 into four principal groups, namely, 

 the eocene, the miocene, the older 

 pliocene, and the newer pliocene, 

 each characterized by containing a 



