THE LOESS OF NATCHEZ, MISS. 313 



Not only do so many of these species occur in the southern 

 part of the Interior Region, but several are restricted to it. 

 Such are Helicina orbiculata, Polygyra stenotrema, Polygyra 

 thyroidcs biiccnlenta, OmpJialina kopnodes, Vitrea placenta/a 

 and Pyramidula alternata costata. Polygyra obstricta and 

 P. inflecta, whose northern range is but little greater, may 

 also be added to this list. The predominance of the larger 

 species of land shells* is also characteristically southern, the 

 Natchez fauna approaching that of the Cumberland sub- 

 region. Moreover, the small form of Polygyra hirsuta, the 

 Polygyra monodon with nearly closed umbilicus, and the large 

 form of Circinaria concava, all suggest a southern origin, the 

 fossils being like the predominating corresponding living 

 forms in that section. 



Only four species of modern molluscs occurring at Natchez 

 were not found as fossils. They are: Polygyra cspiloca (Bid.) 

 Binn., OmpJialina laevigata (Pfr.) Pils., Gastrodonta gularis 

 (Say) Try., and Succiiiea grosvenorii Lea. The first three 

 species are restricted to the southern states. Succiiiea gros- 

 venorii is now locally very common northward to South Da- 

 kota, especially in the country bordering the Missouri. Its 

 center of distribution is far north of Natchez, and its southern 

 extension may be of comparatively recent date. The species 

 is very common in the northern loess along the Missouri river. 



The Natchez fossils bear out the writer's oft-repeated state- 

 ment that the loess fossils of any given region are practically 

 identical with the modern molluscan fauna of the same region. f 

 Indeed, they furnish the most convincing proof of this inter- 

 esting and important fact which has yet been presented. The 

 most characteristic and widely distributed species of the north- 



*See Plate VI. 



TBull. Nat. Hist. St. Univ. Iowa, vol. I, p. 213. — 1890. 



Proc. la. Acad. Sci., vol. Ill, p. 84. — 1896. 



Proc. la. Acad. Sci., vol. V, p. 41. — 1898. 



Proc. la. Acad. Sci., vol. VI, p. l ) l >. — 1899. 

 Jour. Geol. , vol. VII, p. 132. — 1899. 



