194 PROCEEDIXGS OF THE ACADEMY OF [Feb., 



In this area it is not known to vary much east of the Mississippi. 

 Several years ago I received a number of specimens from Arkansas 

 Avhich differed from all previously known in having the whorls so 

 closely coiled as to make the axis quite imperforate, and at the same 

 time the whole shell was flatter. This form I called Gastrodonta 

 hrittsi. Subsequently, specimens from another Arkansan locality 

 came to my hands, in which there was a lamella within the aperture, 

 a structure unknown in the species elsewhere. These became G. 

 demissa var. lamellata. The abundant series collected by Mr. Ferriss 

 since these supposed subspecies were discriminated, shows that in 

 their area they coexist with snails indistinguishable from Eastern 

 G. demissa, and that the variation of that species in this area is not 

 indeterminate, but toward the one or the other of these two diverse 

 modes of modification. 



Polygyra monodon is in the same case. In western Arkansas, 

 three weakly differentiated forms occur, which have been named 

 alici(B, friersoni and imperforata. All are closely related to, and 

 doubtless descendants of, the widespread P. m. jraterna; and all three 

 occur around some localities.^ But in examining several hundred indi- 

 viduals I did not find the variation indiscriminate, even in a small 

 percentage of the snails, but always toward one or the other of the three 

 differentiation-lines signahzed by the three subspecific names, without 

 individuals intermediate between them, or otherwise modified, as tlie 

 species is in some other regions. 



Without further multiplying instances, I may say that while in 

 certain districts, usually of rugged and varied topography, widespread 

 species of snails become locally variable, there has come to my notice 

 but little of the multifarious variation called for by the theory of 

 natural selection. What strikes one is the uniformity with which 

 modifications, varying in amount, yet follow definite paths. 



This impression left by studies of the snails of the United States, I 

 would never have gained from what I have seen of certain Antillean 

 genera, such as Cerion or the Jamaican Urocoptis. In these groups 

 there seems to be great and multifarious variation, without elimina- 

 tion of intermediate forms, thus making specific lines merely arbitrary. 



List of Species. 



Localities marked with an asterisk (*) in this list are given on Mr. 

 Ferriss' authority. 



^ See my former paper on Arkansan snails, Proc. A. N. S. Phila., 1900, pp. 

 -154-456, where the data on distribution are given. 



