P T E 



[372] 



P U M 



mal was capable either of standing 

 firmly on the ground, or of perch- 

 ing upon the branches of trees. It 

 is deemed probable that the ptero- 

 dactyle had the power of swimming. 



PTE'EOPOD. } (from Tnepov, a wing, 



PTEEO'PODA. j and TTOW, a foot, Gr.) 

 The second order of the class 

 Cephalophora. A class of molluscs 

 possessing organs adapted for either 

 swimming or sailing. The genera 

 belonging to this class have the sac 

 formed by the mantle closed on 

 every side ; a structure rendering it 

 necessary that the gills should be 

 placed externally, as regards the 

 sac ; and they are found spreading 

 out like a pair of wings on each 

 side of the neck. This position of 

 the gills, causing them to resemble 

 the wings of an insect, suggested 

 to Cuvier the name which he 

 assigned to the class. 



PTERO'PODOTJS. Belonging to the class 

 Pteropoda ; wing-footed. 



PTEEYG'OTTJS. The name given by 

 M. Agassiz to a genus of fossil 

 fishes discovered in the Upper 

 Ludlow Rock. These singular 

 winged bodies have been called 

 Sera^hims by the Scotch quarry- 

 men. M. Agassiz says, "the more 

 I know of this creature, the more 

 I am tempted to believe that it was 

 a fish ; but how absolutely decide 

 upon it, when we have neither 

 discovered head nor tail, but only 

 large wings." Murchison. Silu- 

 rian System. 



PTILO-DI'CTTA. (from TTTI'XOV, pluma, 

 and SIKTVOV, rete, Gr.) The name 

 assigned by Mr. Lonsdale to a 

 genus of corals found in the Silu- 

 rian rocks, from the feather-like 

 arrangement of the middle and 

 lateral cells, and their net-like 

 union. "This fossil," says Mr. 

 Lonsdale, "is considered by Gold- 

 fuss to be ajlustra, but it is placed 

 by Milne Edwards among the 

 doubtful species of that genus. It 

 differs essentially from Flustra in 



the thickening of the external crust. 

 Prom JSschwra it differs in not 

 having a central partition, and in 

 the surface of the cells not being 

 convex, but depressed as in Plus- 

 tra." For a description of the 

 generic characters the reader is 

 referred to Sir R. Murchison's 

 Silurian System. One species, P. 

 lanceslata, has been established by 

 Mr. Lonsdale. 



PTYCHACA'NTHUS. The name of a genus 

 of ichthyolites of the old red sand- 

 stone. 



PFBE'SCENCE. (from pubesco, Lat. to 

 grow mossy, or hairy.) The downy 

 substance on plants, resembling 

 fine silken short hairs. 



PUBE' SCENT. Clothed with fine short 

 hairs or down. 



PU'DDING-STONE. A conglomerate 

 composed of rounded stones im- 

 bedded in a paste. Pudding-stone 

 is distinguished from breccia by the 

 form of the contained pebbles : in 

 the latter they are sharp angular 

 fragments ; in the former they are 

 rounded nodules. 



PULVINI'TES. (from pulvinus, Lat. a 

 cushion.) A fossil bivalve found 

 in the baculite limestone of Nor- 

 mandy. 



PU'MICE. } (pumex, Lat. pomice, 



PI/MICE -STONE. ) It.) The Bim- 

 stein of Werner : Ponce of Brong- 

 niart : La pierre ponce of Brochant : 

 Lave vitreuse pumicee of Hau'y. 

 A light, spongy, fibrous lava, pro- 

 duced by the action of gases on 

 trachytic and other lavas. The 

 island of Lipari contains a moun- 

 tain entirely formed of white pum- 

 ice : when viewed from a distance, 

 the appearance is that of a moun- 

 tain completely covered with snow; 

 it is called II Campo Bianco. Klap- 

 roth gives the analysis of a speci- 

 men from Lipari as follows : silex 

 76-5, alumine 17' 5, soda and potash 

 5*0, iron 1 75. Immense quanti- 

 ties of pumice are sometimes ejected 

 from volcanoes. "Werner and Kar- 



