1906.1 NATURAL SCIENCES OF PHILADELPHIA, 529 



MOLLUSC A OF THE OZARKIAN FAUNA. 



BY H. A, PILSBRY AND JAMES H. FERRISS. 



In the midst of the valley of the greatest river of the continent the 

 Ozark Mountains stand above the plain. On the east and south they 

 are bordered by lowlands scarcely a hundred feet above sea level, and 

 westward the great plains stretch to the foothills of the Rockies. The 

 Ozarks rise to no considerable height, the highest point, Magazine 

 Mountain, in Logan Co., Ark., attaining to 2,823 feet; but the rough 

 topography, in strong contrast with that of surrounding regions, 

 affords conditions favoring the evolution of special forms, species or 

 races, adapted to these conditions. Many of these forms have prob- 

 ably been evolved just where we now find them, and where they are in 

 a measure isolated by the absence or rarity of similar haunts in the 

 lower and more level country surrounding the mountainous area. 



Some aquatic forms, Unionidce and Pleuroceratidce, are also peculiar 

 to the Ozark region, though in the main Mississippian species rule. 



During the greater part of March and April, 1903, the authors col- 

 lected at numerous places in the western Ozarks, in southwestern Mis- 

 souri, western Arkansas, and Indian Territory, the localities selected 

 being supplemental to those worked by Mr. Ferriss in 1900 and 1901. 

 The results of our work are herein dealt with. 



I. Faunal Relations of the Ozark Region. 



The Alleghanian (= Transition of Merriam), Carolinian (= humid 

 Upper Austral) and Austroriparian ( = humid Lower Austral) zones as 

 mapped by Dr. Merriam,^ in eastern North America, are probably 

 reflected as clearly in land molluscan distribution as in that of verte- 

 brates and plants, though of course there is broad overlapping of 

 faunas, and the smaller details of the zonal limits remain to be deter- 

 mined by diligent local work. In the Northern States, this local 

 amplification may well be based upon Dr. Merriam's map; but in the 

 Southern Appalachian system some significant discrepancies appear 

 when the land moUusks are compared with vertebrate distribution. 



^ Life Zones and Crop Zones of the United States, Bulletin No, 10, U. S. De- 

 partment of Agriculture, Division of Biological Survey, 1898. 



