1903.1 NATURAL SCIENCES OF PHILADELPHIA. 193 



MOLLUSCA OF WESTERN ARKANSAS AND ADJACENT STATES, WITH A 

 REVISION OF PARAVITREA. 



BY HENRY A. PILSBRY. 



Early in 1901 Mr. J. H. Ferriss explored for mollusks a portion of 

 southwestern Missouri, western Arkansas, and some adjacent locali- 

 ties in the Indian Territory, the region covered lying mainly north of 

 that exploited in 1900.^ 



The rugged topography of a portion of this region, and its elevation 

 above the plains on all sides, have resulted in the evolution of many 

 species and subspecies special to the tract. The general conditions 

 of life are varied from those prevailing over the comparatively level 

 States along the Mississippi, and with this change has come readjust- 

 ment on the part of the snails. The mode of this readjustment I hope 

 to study further when more material is available. From the data pre- 

 sented in this paper and my article of 1900 on the same fauna, it appears 

 that — 



(1) Species having a wide geographic range become much more vari- 

 able in this district. 



(2) Variation is usually not indiscriminate, hut in the large majority 

 of individuals follows one or two definite lines of deviation from the "nor- 

 mal" or widespread form. 



(3) Various specific stocks show various degrees of deviation or 

 differentiation from their widespread "normal" forms. 



The first of these propositions is exemplified by a large nmnber of 

 species, and would be noticed by any one possessing the material for 

 comparison. The second point is one of some importance. It is 

 illustrated likewise by many species, of which several may be taken 

 as examples. Gastrodonta demissa is a snail ranging from "Western 

 Pennsylvania to Georgia, west to Arkansas and Eastern Texas. "- 



1 See Pilsbry in these Proceedings for 1900, pp. 449-459, and especially Ferriss. 

 Nautilus, XIV, pp. 25-31, July, 1900, where the country is described, and manj^ 

 species not in other lists are noted. Information bearing on the molluscan 

 fauna of adjacent districts may be found in the following papers: F. A. Sampson, 

 "Shells of Pettis county, Mo.," in Bull. I, Sedalia Natural History Society (1885) : 

 "Preliminary List of the Mollusca of Arkansas," Ann. Rep. Geol. Survey of 

 Arkansas for 1891, Vol. II, pp. 179-199 (1893); C. T. Simpson, "Notes on Some 

 Indian Territory Land and Fresh-water Shells," in Proc. U. S. National Mu- 

 seum, 1888, pp. 449-454. 



■^ Pilsbry, Catal. of the Land Shells of America, p. 28 (1898). 



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