102 
IOWA ACADEMY OF SCIENCES. 
surface distribution of the modern shells. If the loess was not 
deposited in toto 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 part 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 Polygyra inultilineata 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 Blufis 
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. 
In both the loess in the east and the west^" SiPiyradmm 
edeiitulum alUcola, Pyramiclula strigosa ioivensis, f Succinea grosven- 
orii , — forms belonging now to the dry western plains, — are 
*The loess hereia designated as “ eastern ” is that of eastern Iowa,— the “ western ” 
being that of western Iowa and eastern Nebraska. 
+This form has heretofore been reported as var. cooperi which' lives abundantly in 
the far west, but Pilsbry regards it as extinct and distinct, and has described it under 
the name iowensis. All living forms of strigosa 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. 
