Geographical Distrihution of Certain Fresh- Water Mollusks. 317 



fig. 6a, is incorrect, in this, that it shows more concentric lines near 

 the beak than appear on the specimen. 



Length, 6-10 inch ; height, 28-100 inch ; thickness, 18-100 inch. 



This species may be readil}^ distinguished from others in this genus 

 b}" its small size, strong, concentric lines, and thin, sharp, prow-like 

 projection of the anterior end. 



I collected this species in the lower part of the Hudson River Group, 

 at the excavation for Columbia Avenue, in Cincinnati, at an elevation 

 of about 140 feet above low-water mark of the Ohio river. 



ON THE GEOGBAPHIGAL DISTRIBUTION OF CERTAIN 

 FRESH-WATER MOLLUSKS OF NORTH AMERICA, AND 

 THE PROBABLE CAUSES OF THEIR VARIATION. 



By A. G. Wetherby, A.M., 



Prof, of Geology and Zoology, University of Cincinnati. 



Few subjects connected with the study of plants and animals have 

 presented questions of greater interest than that of their geographical 

 distribution. To work out these problems, naturalists have consented 

 to absent themselves from civilization and home for years ; to wander 

 through the deserts of the semi-tropics, the thick jungles and forests 

 of equatorial regions, and over the ice-floes and glaciers of the ultimate 

 attainable polar climes ; and from these extremes, whatever the un- 

 tiring vigilance of trained workers in special fields has been able to 

 glean, the}^ have brouglit together, analyzed, compared, and so set 

 in order, as to bring the earth's wide spread glory of organic life into 

 something approaching a comprehensive system. 



The result of this labor has been to show that species are not the 

 unchanging certainties that the earlier students of the earth's races 

 believed them to be, that„in fact, the word species is little less than 

 "a convenient abstraction" by which we separate from the multitude 

 of life around us, certain individuals haAnng common characters over 

 limited areas. 



The statement of Forbes, that "every true species presents in its 

 individuals certain features, specific characters, which distinguish it 

 from eveiy other species, as if the Creator had set an exclusive mark 

 or seal on each t3q)e," can not now be used in the sense set forth, unless 



