28 THE NAUTILUS. 



the species disappears with it, and so we have a large list of descril ed 

 species in our literature with but few living forms to represent them. 



To simplify the study of the genus I discard all sub-genera and 

 divide the genus into species, varieties and forms. 



Species are forms having some characteristics which remain con- 

 stant for ages. 



Varieties are forms having the ruling characteristics of the species 

 to which they belong, with modifications that remain constant for 

 years. 



Forms show temporary variations caused by environment or 

 hybridization, and are transient. New forms are appearing every 

 year. I would not attempt to refer all the living forms to their 

 respective progenitors, but leave the collector to attach the label to 

 suit his own taste. 



Where a form can now be found that substantially bears the 

 description given forty or fifty years ago, it should be considered a 

 species, if originally founded on sufficient differences from other 

 described species to entitle it to rank as such ; but where no specimens 

 of a species has been found since it was described many years ago, it 

 is likely to have been only a local form, and to have become extinct. 



Many of the early described species are found now bearing the 

 same distinctive characters as when first described. It is these typ- 

 ical forms that must be our guide, and not the varieties. 



The result of my examination of all the described species occurring 

 east of the Rocky Mountains will be found in the following paper. 



Physa heterostropha Say. 



Synonyms : P. Pliilippi Kiister, P. plicata, De Kay. 



This species was described by Thomas Say in 1817 from forms 

 taken in the Delaware River, and the same forms are found there 

 yet. The water having become polluted, they are covered with a 

 dark coating and many of them distorted. The type is not inflated 

 nor elongated, yet the larger part of the specimens of the species now 

 accepted by conchologists are more or less inflated. Some even go 

 so far as to include under this head all the varieties of P. ancillaria, 

 Say, P. lordi Baird, and P. sayii Tap pan. While there are some 

 good reasons for bunching all the American Physee into one species 

 with a vast number of varieties, I do not believe such a course would 

 be conducive to the best interests of science. Species should be 



