Geology and Mineralogy. 549 



examples have been invariably extreme instances, however ; the 

 less striking ones pass unnoticed, since hardly a lake, pond, marsh, 

 slough, stagnant stream, semi-estuary, or enclosed or partially 

 enclosed body of water, contains molluscs which are not more or 

 less subject to these aberrancies. Frequently, particularly in the 

 arid regions of Western North America, these forms have been 

 redescribed as separate species, but in no instance are the charac- 

 ters inherited, though the stock may have passed through a long 

 line of abnormal generations. The progeny under such condi- 

 tions appear to be unusually liable to become abnormal likewise, 

 but this may be explained on the basis of hereditary suscepti- 

 bilitjr. . . . 



" Forms produced under these circumstances are legion, but 

 appear in every instance not as possessing new characters, but the 

 result of an accentuation of the principal environmental and evo- 

 lutionary influences which affect the mollusc, hence the term 

 syntonia. . . . 



" The thickness of the shell normally depends, within limits, 

 on the amount of lime in the water. . . . An accentuation of 

 the evolutionary influences in the Gastropoda may produce an 

 exaggeration of the rest periods, resulting in the development of 

 more or less regular costae, of the spiral striae causing malleations, 

 angulations, or keeling, of the inflation of the aperture, ... an 

 unnatural development of the columnar fold ... and the pro- 

 duction of irregularities of growth. . . . The Pelecypoda appear 

 to be rather less susceptible, but arcuity among the Unioids is 

 generally due to this cause. . . . The only salts which occur 

 widespread or in sufficient abundance to be regarded as probabil- 

 ities are those of sodium, potassium, calcium, and magnesium. 

 Sodium and potassium salts, known commonly as white and black 

 alkali respectively, are frequently abundant in the arid regions of 

 the west. It has been repeatedly observed that one or the other 

 or both may be present in such abundance as to cover entirely 

 the ground in the vicinity of a pond or stagnant stream while the 

 Mollusca are indifferently normal or affected, and when affected 

 seldom to the degree one would expect if the distortion could be 

 the result of the salt in question. Calcium salts, as is well known, 

 have no deleterious effect upon molluscan life, but are a prime 

 necessity for its existence. Magnesian compounds, on the other 

 hand, produce remarkable physiological effects and act as poisons. 



" Reasoning on this basis the writer has undertaken a series of 

 experiments with balanced aquaria which prove beyond doubt 

 that the small quantities of magnesium salts ordinarily present in 

 stagnant wp.ter produce these puzzling forms, and once produced, 

 their results are not readily overcome. Both the sulphate and the 

 chloride appear to be equally pernicious. 



" Whether or not other salts have similar effects has not been 

 ascertained in every case. The eight or nine commoner ones in 

 ordinary water produce no appreciable distortion" (114-116). 



c. s. 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 209.— May, 1913. 



38 



