102 IOWA ACADEMY OF SCIENCES. 



surface distribution of the modern shells. If the loess was not 

 deposited in fofo at once, and this seems to be conceded, there 

 were successive land-surfaces upon portions of which shells 

 grew. These shells varied from time to time in numbers — some 

 persisted during long periods, some disappeared and others 

 took their places. If we study the vertical distribution of the 

 fossils in the loess the same variation in the succession of 

 species is observed. Some species occur throughout the thick- 

 ness of a particular exposure, but more frequently a i^art of 

 the loess is without fossils; certain species occupy a part of 

 the deposit, — while above or below them are other species, — as 

 though the varying generations of surface species had been 

 successively buried in the deposit. The number of specimens 

 upon any one of the successive land surfaces was not very 

 great even in richly fossiliferous loess, for if we draw lines 

 approximately parallel to the present surface to represent the 

 successive surfaces, we will find that in any one of them but 

 few fossils occur. 



Where depauperation or variation in size is noticeable in the 

 fossils, it will be found that it takes place in the direction of 

 the western modern forms. For example, while the common 

 modern Polygi/rd iiiultilineata at Iowa City is large, the common 

 fossil form is small,^ — but the small modern and the large fossil 

 forms are also occasionally found, — but not respectively with 

 the preceding forms. On the other hand, at Council Blulfs 

 and Omaha, the modern shells of this species are usually 

 small, like those of the loess, though both fossil and modern 

 shells of the large type occasionally occur. Thus the fossils 

 of this species, from the eastern part of the state, resemble 

 both the fossil and modern shells from the western part. 

 Succinea avara is another example. The small typical form is 

 common in the loess at Iowa City, but the modern shells are 

 not frequent, occurring always on more or less wooded hill- 

 sides,^ — while westward the type is the common modern form. 



la both the loess in the east and the west* Spliyradiuin 

 edentuhnn alticola, Pyrainklula strigosa ioicenfiis,^ Succinea grosven- 

 orii, — forms belonging now to the dry western plains, — are 



*The loess hereia deslg-natefl as " eastera " is that of eastern Iowa,— the " western " 

 being- that of westera Iowa aod eastern Nebraska. 



+This form has heretofore been reported as var. conperi whicli lives abundantly in 

 the far west, but Pilsbry regards it as extinct and distinct, and has described it under 

 the name inwensis. All living- forms of strigom belong to the high, dry regions of the 

 west. Neither of these species was found at Council Bluffs, but both are found in the 

 loess of Nebraska. Sphyradium was formerly included in Pupa. 



