116 PROCEEDINGS OF THE MALACOLOGICAL SOCIKTY. 



such abundance as to cover entirely the ground in the vicinity of 

 a pond or stagnant stream wliile 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 moUuscan life, 

 but are a pi'ime necessity for its existence. Magnesium 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 

 water 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. 



SYNOPSIS OF SPECIES. 

 Having considered briefly the faunal subdivisions, origin and 

 development of the fauna, method of classification, and the interfering 

 factor, syntonia, it is now possible to proceed intelligently with an 

 outline of the classification and distribution of the aquatic Mollusca of 

 the Californian Province. It may be noted at the outset that the 

 following pages are intended chiefly as a working nucleus for future 

 papers. A monograph of the fauna would require as many years for 

 its preparation as this synopsis has months, to say nothing of the 

 necessity of far more extensive field-work, hence its deficiencies may 

 be to a degree pardonable. 



PELECYPODA. 



Superfamily UXIONOIDE^ (Swainson), 1840. 



The ancestral form of the Naiad shell seems to have been heavy, 

 quadrate-discoidal, zigzag sculptured, and possessing a broad, coarse 

 hinge. From this type there has been a general tendency for the 

 more specialized forms to acquire a smooth, thin, posteriorly elongated 

 shell, the markings being carried back to the umbones forming the 

 characteristic beak sculpture and towards a reduction in the breadth 

 of the hinge, followed by a loss first of the lateral teeth, later the 

 pseudo-cardinals, with the ultimate result of a general simplification 

 of all the shell parts. 



This paterina stage is not represented without some modification 

 by any living species known to the writer. Probably the nearest 

 approach is in such types as Jlyria corrugata. Quadnda undidafa, and 

 Rotundaria tuherculata, in which the sculpture has been largely carried 

 back to the umbones, a few atavistic, undulating, or broken pustulose 



' It is probable since the salts are ionized in solution that the hydrochloric 

 acid present in the salivary juices would change the sulphate to chloride 

 before it entered the circulatory system. 



