HANNIBAL: CALIFOKNIAN FriESHWATER MOLLUSCA. 115 



susceptibility, much as tuberculosis passes from one generation to 

 another in the human race. 



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 evolutionarj^ influences 

 which affect the mollusc, hence the term s>/ntonia suggested to the 

 writer by Dr. David Starr Jordan. 



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

 the amount of lime in the water. Affected by these influences the 

 variation increases, resulting in such extreme forms as typical LijmncBa 

 auricular ia on the one hand and the so-called Mighelsi on the other. 

 L>jmncea jJahistn's commonly ranges from 20 to 30 mm. in altitude, 

 depending chiefly on the rapidity of flow of the water. Syntonic 

 forms have been noted oOmm. in altitude. OnUnarilj Fhi/sa fo?itinal'is 

 varies from about 60° to 70° in the breadth of the apical angle 

 according as it is found respectively in running streams or lakes.* 

 In aberrant forms the breadth may be increased to 95°. 



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 

 (to which cause is due the remarkable typical form of Zi/mnaa 

 auricularia, a particularly susceptible species, of which the so-called 

 peregra and catascopium represent the normal condition), an unnatural 

 development of the columnar fold as frequently happens in species 

 belonging to typical Lymnaia, and the production of irregularities of 

 growth such as are common in Planorbis trivolvis. The Pelecypoda 

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

 generally due to this cause. 



No two species of Mollusca are affected to a like degree or exactly 

 in the same manner. Lymncea, Physa, and Anodonta, for instance, 

 become progressively deformed, and senile individuals show the effects 

 most markedly. On the other hand, in Ambloxus it is almost impossible 

 to obtain normal adolescent specimens of certain species, whereas the 

 adults are but rarely aberrant. 



The cause of these phenomena has been the source of much 

 discussion. Dall held volcanic dust responsible among the Galapagos 

 Bulimuli, and the salts concentrated in the receding waters 

 of Lake Bonneville in the instance of the Quaternary freshwater 

 species of Utah. Cooke suggested brackish water to account for the 

 deformation of the Lymnaeas from the Aral Sea. No one, however, 

 has isolated the particular salt which it is evident is producing the 

 mischief. The only salts which occur widespread or in sufficient 

 abundance to be regarded as probabilities 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 



* Gray's Turton, p. 228, 1857. 



