408 PROCEEDINGS OF THE ACADEMY OF [1896. 



regions with the presence of some alkaline salt, which accentuates the 

 action of those factors in the organism which are concerned in the 

 formation of the minor irregularities of the shell surface. The man- 

 ner in which this is brought about is one of the prettiest illustrations 

 of the direct action of the environment which I know, and seems to 

 be sufficiently established by both geological and physiological evi- 

 dence. 



In the arid region of the far west, especially in the desiccated lake 

 basins of Utah, Nevada and California, it has long been observed by 

 the writer, Dr. R. E. C. Stearns and others, that in the successive 

 beds of fresh water marl, which the now dried up lakes deposited in 

 Pliocene and Pleistocene times, the shells indicate a progressive 

 change in surface characters as the alkalinity of the water increased, 

 until at last the amount of alkali became so great that the mollusks 

 were exterminated or found a precarious refuge in the fresh water 

 streams which fell into the basins in question. The shells, without 

 regard to genus or systematic relations, showed a unanimous ten- 

 dency to become ridged, plicated or rugose ; the regularity of the 

 gastropod coil was interfered with, abnormalities became more com- 

 mon, and, toward the last, almost general. Projecting sculpture, 

 spiral threading, carinse, riblets, etc., were exaggerated : size gener- 

 ally diminished, the height of the spire relatively to the diameter 

 became less, and general degeneration curiously combined with ex- 

 treme accentuation and irregularity of surface characters. Some- 

 thing of the same sort is visible at the present time in the shells of 

 fresh water gastropods in the irrigating ditches of farms in the 

 alkaline arid region ; those shells, in the ditches where the water 

 has leached out alkaline matter from the soil, showing evidences of 

 change in the same direction in surface sculpture, as I have person- 

 ally observed in the Honey Lake Valley, Nevada. 



In Whitfield's observations on the degeneration of Limncea mega- 

 soma — kept for many generations in an aquarium where the water 

 lost by evaporation was constantly replenished, the old residual sup- 

 ply not being emptied, so that a concentration of the salts contained 

 in the much greater bulk of the original water took place in the 

 aquarium — somewhat analogous but less marked changes are re- 

 corded. 



The dynamical origin of these changes may be explained by con- 

 sidering the origin of the surface characters of the shell. The de- 

 position of the shell substance and epidermis takes place from the 



