IOWA ACADEMY OF SCIENCES. 41 



thought that the remains now fossil drifted in from very 

 remote regions. Land shells are imbedded to-day in fine 

 alluvium along our streams,* but these are seldom carried to a 

 distance, but are rather covered in situ by the fine silt of the 

 stream. The above assumed conditions, however, would call 

 for the transportation of the materials from great distances, 

 and the larger shells would be precipated long before the fine 

 silt had reached its destination, or if tioating, would be thrown 

 ashore in bands. Any observer who collects shells in river 

 drift and then traces the rarer species of more restricted distri- 

 bution to their source, can ascertain that these shells are not 

 carried far. Dredging in "sloughs," ponds, and sluggish 

 streams also shows that land shells are seldom washed into 

 them. 



Moreover the molluscan loess-fauna of any region is on the 

 whole like the modern fauna of the same region. For example, 

 Binney reports a number of species from the "post-pleiocene " 

 (evidently the loess) of the lower Mississippi valley, of which 

 elevenf are southerly species, and all now live in the same 

 region. Call reportsj fifteen species from the loess of Arkansas, 

 three of them included in the southerly list in Table II, and all 

 belong to the modern molluscan fauna of that state. The same 

 is true of the faunas of Iowa and Nebraska as has been stated. 



This does not indicate transportation from a great distance. 

 It is interesting and noticeable that for the most part the spe- 

 cies of the loess are common over the same region now. There 

 are some exceptions, for there have been changes no doubt, 

 but these changes, as indicated by the distribution of the shells, 

 are no greater than may now be observed in any limited 

 region in the course of a few years. Species are sometimes 

 disposed to appear, disappear, and reappear in a surprising 

 manner in a given locality, and, if we may judge from the ver- 

 tical distribution of the fossil shells, the same was true during 

 the deposition of the loess. 



The horizontal distribution of the fossils is likewise such 

 that it suggests at once that they were deposited in situ. § 



As there are surface areas to-day which have no molluscs, 

 lying in close proximity to those on which molluscs are abun- 



*For author's account of an example see Bull. Lab. Nat. Hist , State Unlv of Iowa, 

 Vol. II, pp. 169-174. 



tSee Table II. 



tGeol. Sur. Ark., Vol. II, pp. 49, 165, and 166. 



§For a further discussion of this point by the author ses Bull Lib. Nat. Hist. 

 State Univ. of Iowa, Vol. Ill, pp. 95-96. 



