PLAC01D FISHES 



the action of rivers which may still 

 exist, whilst the latter have been 

 formed by the action of extinct or 

 ancient rivers ; in either case the 

 contained mineral is derived from 

 the disintegration of metalliferous 

 veins (primary deposits). 



Deep-lying alluvials or placers 

 are worked by ordinary methods 

 applicable to the exploitation of 

 bedded mineral deposits, such as 

 coal. Superficial deposits are 

 worked by a variety of methods. 

 Panning is carried out by hand in 

 a pan about 16 ins. diameter, the 

 gravel being washed away by water, 

 the mineral, being heavier, remain- 

 ing in the pan. See Hydraulick- 

 ing ; Gold ; Mining. 



Placoid Fishes (Gr. plax, flat 

 plate). Name sometimes given to 

 the Elasmobranchs, a group of 

 cartilaginous fishes which includes 

 the shark, dog-fishes, and rays. 

 See Elasmobranch. 



Plagioclase. In geology, a 

 group of triclinic felspars. The 

 plagioclases form the chief con- 

 stituents of igneous rocks, and 

 appear also in many metamorphic 

 rocks. The rocks of this group are 

 described in this Encyclopedia 

 under their respective names, e.g. 

 albite, andesine, anorthite, labra- 

 dorite, oligoclase, etc. They are 

 valuable in the study of. rocks, 

 their optical properties serving as 

 an index for the study of thin 

 sections of rocks of all kinds under 

 the microscope. See Felspar. 



Plague (Lat. plaga, stroke). 

 Term formerly applied to epidemics 

 of various diseases, but now re- 

 stricted to the disease caused by 

 the bacillus pest is. The disease has 

 been known in epidemic form at 

 least from the 6th century. In the 

 14th century a devastating out- 

 break known as the Black Death 

 occurred in Europe, and is esti- 

 mated to have swept away one- 

 fourth of the population. In the 

 Great Plague of London, 1665-66, 

 some 70,000 persons perished. 

 Since that date epidemics in 

 Europe have declined. In 1894 

 there was a severe outbreak at 

 Hong Kong, following which the 

 disease spread through large areas 

 of India, China, Japan, Australia, 

 South America, the West Indies, 

 Madagascar, Egypt, and Russia, 

 while cases occurred in Marseilles, 

 Hamburg, and Glasgow. At the 

 present day the regions in which 

 the disease is endemic are Meso- 

 potamia, Northern India, Tibet, 

 and Yunnan in China. 



Plague is, in the first instance, a 

 disease of rats, and the infection 

 is conveyed from the rat to man 

 solely by the rat flea, which, after 

 biting an infected rat, bites a 

 human being. The disease may be 



6183 



conveyed from place to place by 

 rat fleas carried in merchandise or 

 on human beings. Four main forms 

 of the disease are recognized. The 

 ambulatory type is the mildest 

 form, the symptoms being rise of 

 temperature, lasting for a few 

 days, and swelling of the glands of 

 the groin, which may pass on to 

 suppuration. 



The bubonic form constitutes 

 some three-quarters of all cases. 

 The initial symptoms are headache, 

 pain in the back, stiffness of limbs 

 and restlessness. The temperature 

 rises and severe prostration occurs. 

 Death may take place at this stage, 

 but in most cases swellings known 

 as buboes appear, affecting the 

 glands of the groin, the axilla, and 

 the neck. These usually suppurate. 

 Haemorrhages under the skin may 

 occur ; these were known as 

 " plague spots " in the Middle 

 Ages, and gave the disease its 

 name of Black Death. The 

 patient may become delirious and 

 death occurs from the third to the 

 fifth day. In favourable cases the 

 temperature declines gradually, 



Plaice viewed from above, showing 

 position of both eyes on upper side 



but convalescence may be pro- 

 longed owing to the suppuration of 

 the buboes. In the septicemic type 

 the signs of blood poisoning are 

 very severe, and the patient may 

 die in from three to four days, 

 before the buboes have appeared. 

 In the pneumonic form of plague 

 shortness of breath, cough, and 

 pain in the chest are among the 

 early symptoms, and signs of 

 pneumonia develop. Death may 

 occur as early as 16 hours after the 

 appearance of the symptoms. 



No drug has any appreciable effect 

 on the disease, but a serum has 

 been prepared which is beneficial in 

 some cases. High fever may be 

 treated by cold sponging. Preven- 

 tion of plague is an important 

 necessity in areas liable to be in- 

 fected. The most essential step is 

 to take energetic measures against 

 rats by their systematic destruc- 

 tion. Houses should be made rat- 

 proof. Ships arriving at port should 

 be fumigated to destroy rats. The 

 risk of acquiring the disease in 

 infected areas has been reduced by 

 means of Haffkine's prophylactic 

 vaccine. See Black Death ; Great 

 Plague ; Rat. 



PLAIN 



Plagues of Egypt. Ten plagues 

 brought upon the Egyptian 

 Pharaoh and his people by 

 Jehovah, when the Israelites were 

 hi bondage in Egypt (Exod. vii, 

 14 xii). The plagues were turn- 

 ing of the waters of the Nile to 

 blood ; swarms of frogs, of gnats, of 

 flies ; affliction of cattle with pesti- 

 lence ; of man and beast with boils 

 and blains ; havoc by thunder 

 and hail ; swarms of locusts ; dense 

 darkness ; and, the last and most 

 terrible, the death of the first-born 

 of man and beast. 



Plaice (Pleuronectes platessa). 

 Important food fish of the family 

 Pleuronectidae. A native of the 

 Atlantic and North Sea, from Ice- 

 land and N. Europe, to the Bay of 

 Biscay, but not on the American 

 coasts, it is also found rarely in the 

 Mediterranean. Of a long, oval form, 

 the body of the plaice is strongly 

 compressed from side to side, and 

 fringed with the long dorsal and 

 ventral fins, which extend almost 

 from head to tail. The left side is 

 white without any markings, but 

 the right, or upper side, is fully 

 coloured brown, on which are 

 scattered spots of orange or red. 



Beginning life as a " round " fish, 

 like the cod, the young plaice soon 

 takes to lying on the bottom on 

 its left side, and the left eye travels 

 round to the right side. The young 

 flat-fish affects sandy shores in shal- 

 low water, going deeper as it gets 

 larger, until as an adult it seeks the 

 spawning grounds in from ' 10 to 40 

 fathoms. It is adult when from 2 

 to 3 years old, and 10 to 12 ina. 

 long, but if it escapes the trawl may 

 grow to 24 to 30 ins. Its principal 

 food consists of small molluscs, 

 sand stars and marine worms The 

 average egg-production of a female 

 plaice is a quarter of a million, 

 though nearly twice this number 

 have been recorded. The eggs, 

 which measure ^th of an inch, 

 float a little below the surface. 

 See Fisheries. 



Plaid (Gael, plaide, blanket). 

 Strip of cloth two yards broad and 

 four yards long, once used as a 

 garment by Scotsmen, and known 

 as the breachan feilidh. See High- 

 land Dress ; Tartan. 



Plain (Lat. planut, flat). Area 

 of flat or approximately flat 

 land. Plains may be classified 

 according to their formation, as 

 plains of accumulation and plains 

 of erosion. 



Plains of accumulation are built 

 up of sediment which forms hori- 

 zontally bedded layers of rocks, so 

 that in these plains the surface cor- 

 responds to the structure. The 

 sediment brought down by rivers, 

 or produced by wear and tear of 

 the coast, is spread over the floor 



