30 THE CHICAGO ACADEMY OF SCIENCES. 



the series, molluscan life disappears altogether. The evaporation of 

 the water caused an increase in its alkalinity, and this condition of the 

 environment is reflected in the shell, which becomes more solid, smaller, 

 and ribbed or corrugated. 1 



Some years ago Dr. W. H. Dall 2 penned the following lines, which 

 admirably describe the effect of unfavorable environments on fresh- 

 water shells : 



"The extra development of plicate sculpture is generally associated 

 in arid regions with the dryness, and in moist 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 irregu- 

 larities of the shell surface. The manner in which this is brought about 

 is one of the prettiest illustrations of the direct action of the environ- 

 ment which I know, and seems to be sufficiently established by both 

 geological and physiological evidence. 



"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 extermi- 

 nated 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 tendency to become ridged, 

 plicated or rugose ; the regularity of the gastropod coil was interfered 

 with, abnormalities became more common, and, toward the last, almost 

 general. Projecting sculpture, spiral threading, carinse, riblets, etc., 

 were exaggerated ; size generally diminished, the height of the spire 

 relatively to the diameter became less, and general degeneration curi- 

 ously combined with extreme accentuation and irregularity of surface 

 characters. Something of the same sort is visible at the present time 

 in the shells of f resh-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 per- 

 sonally observed in the Honey Lake Valley, Nevada. 



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

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



!See Call. Bull. U. S. Geol. Surv.. no. 11: Stearns, Proc. U. S. Nat. Mus. 

 XXIV, p. 271. 



2 Proc. Phil. Acad., 1896, pp. 407-409. 



