126 B. SHIMEK 



are abundant one exposure contains species of one kind, another 

 near by presents a new, or at least a different list, while still 

 another has none — and the same variation which may be 

 observed in the local distribution of the recent shells in any 

 restricted locality, will be exhibited in individual exposures of 

 fossiliferous loess. 



In horizontal distribution the fossils show the same mode of 

 distribution as that already noted in the modern forms. The 

 specimens are not heaped together, but are scattered about like 

 the modern shells, usually a number of species mingled together, 

 but in unmodified loess invariably not crowded, so far as the 

 writer's observations have gone. 



The vertical distribution of the fossils also conforms to the 

 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 number, 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 thickness of a par- 

 ticular exposure, but more frequently a part of the loess is with- 

 out 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 fos- 

 siliferous 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 mod- 

 ern Polygyra multiline ata at Iowa City is large, the common fossil 

 form is small, though the small modern and the large fossil 

 forms are also occasionally found, but not respectively with the 



