PET 



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PET 



PETE'AIA. A genus of fossils found 

 in the Silurian formation, of which 

 many species have been described. 



PETRI'COLA. (from TTGT/JO?, a stone, 

 Gr. and colo, to inhabit, Lat.) A 

 genus of transverse, inequilateral, 

 bivalve, marine shells, belonging 

 to the family Lithophagi. Two 

 muscular impressions ; two hinge- 

 teeth on one valve, and a bifid one 

 on the other ; anterior side rounded, 

 posterior side more or less slightly 

 gaping ; ligament external. Petri- 

 colse are found at depths varying 

 to ten fathoms ; they inhabit cavi- 

 ties, of their own working, in rocks 

 and shells. They may be distin- 

 guished from saxicava by the regu- 

 larity of their form, and by the 

 teeth on the hinge, which in saxi- 

 cava become obsolete when the 

 animal is full grown. 



PETEIFA'CTION. (petrifaction, Fr.) 



1 . A substance converted into stone. 

 Substances, either animal or vege- 

 table, converted into stone by the 

 infiltration, or incrustation, of sili- 

 cious matter. 



2. That process by which organic 

 remains are mineralized, and their 

 external form, and more or less of 

 their internal structure, preserved. 



PETEE'SCENT. (from petrescens, Lat.) 

 Becoming stone; growing into stony 

 matter. 



PE'TEIFIED KAIL. A local name given 

 to a bone of the Asterolepis, a gi- 

 gantic ganoid of the old red sand- 

 stone. This nail-like bone formed 

 a characteristic portion of the As- 

 terolepis. Hugh Miller. 



PETRO'LEUM. (petr6le,'Fr.) A mineral 

 oil, rather thicker than tar, and of 

 a reddish-brown colour ; it has 

 obtained its name from the circum- 

 stance of its oozing out of rocks 

 like oil. In the East it is burnt as 

 oil. It is unctuous to the touch, 

 and exhales a strong and unpleasant 

 odour. It is lighter than water, 

 its specific gravity being 0'87. In 

 the island of Zante, petroleum is at 



the present time obtained from the 

 same spot, and in the same manner, 

 as in the days of Herodotus. The 

 most powerful springs producing 

 petroleum are on the Irawaddi, in 

 the Burman empire. In one lo- 

 cality there are said to be 520 

 wells, yielding annually 400,000 

 hogsheads of petroleum. It occurs 

 in most countries where coal is 

 found. 



In describing the coal-field of 

 Coal Brook Dale, Sir R. Murchison 

 says, "Petroleum is of constant 

 occurrence in the upper as well as 

 lower measures; the chief source 

 of this mineral at Coal Port, which 

 formerly afforded one hogshead a- 

 day, being in a thick-bedded sand- 

 stone of the upper measures. This 

 supply has, however, much de- 

 creased with the opening of new 

 coal works. Other wells have 

 been discovered in the lower coals 

 at Prior's Lee. In some pits, the 

 petroleum exudes in such quantities 

 that the works are necessarily 

 boarded up to prevent its infiltra- 

 tion upon the workmen." 



PETEO'LOGF. The study of rock 

 masses ; or the examination of 

 those characters, structures, and 

 accidents of rocks which can only 

 be studied on the large scale, and 

 only be observed in the field. 

 Jukes. 



PETEO-PHILOIDES. The name assigned 

 by Mr. Bowerbank to a genus of 

 fossil fruits found in the London 

 clay : he thus describes the generic 

 characters. "Fruit, a strobilus. 

 Squamae usually confluent, rarely 

 separate. Seed bilobate, without 

 a comose or foliaceous appendage." 

 Mr. Bowerbank describes seven 

 species, and in speaking of the 

 genus he says, alluding to their 

 approximation to Petrophila, "I 

 have thought it better to call them 

 Petro-philoides, rather than desig- 

 nate them by any other name 

 which would not serve as an indi- 



