Apeil 4, 1913] 



SCIENCE 



505 



to light only four of these species, repre- 

 sented by very few individuals, which had 

 dropped to the bottom of the bay. Ter- 

 restrial shells which drift into water are 

 soon cast ashore. 



Nearby wooded slopes frequently con- 

 tribute terrestrial shells to ponds and 

 streams, but the latter invariably supply a 

 relatively large number of shells of aquatic 

 species. 



It is also noticeable that such species as 

 Zonitoides nitidus and Succinea retusa, of 

 low grounds, and Helicodiscus parallelus, 

 Vitrea hammonis, Bifidaria armifera, B. 

 contracta, B. pentodon, Zonitoides arbo- 

 reiis and Z. mmusculus, which are common 

 on timbered alluvial flats and which occur 

 so frequently in alluvium and modern 

 river drift, are relatively very rare in ter- 

 restrial deposits like the loesses. 



Another method by which terrestrial 

 shells sometimes find their way into al- 

 luvial deposits has been observed where 

 shallow ponds become dry during the sum- 

 mer. These ponds usually contain aquatic 

 pulmonates, and their shells remain in the 

 alluvium of the pond. Terrestrial species 

 then creep out over the exposed surfaces, 

 not infrequently leaving their shells to 

 mingle with those of the pond species. The 

 subsequent flooding of the pond results in 

 the inclusion of both types in the alluvium 

 of the pond. In the alluvium of one pond 

 of this type between the Lakes Okoboji, 

 Iowa, 12 aquatic and 5 terrestrial species 

 were found, the latter being represented 

 by few individuals. 



StiU another method by which aquatic 

 species may be buried at altitudes higher 

 than the level of the free water surface 

 may be observed in seepy places fed by 

 permanent springs. Such spots are not 

 uncommon even on higher slopes and they 

 frequently contain aquatic species belong- 

 ing to the genera Pisidium, Lymncea and 



Physa, probably introduced by smaU 

 wading birds. Terrestrial species from 

 nearby surfaces may creep out or be car- 

 ried into the bog and the shells of both will 

 then be mingled in the deposit of the bog. 

 In one of these small bogs near Council 

 Bluffs, Iowa, located at an altitude fully 

 twenty feet above the high-water level of 

 the Missouri River and closely surrounded 

 by a forest, 5 terrestrial and 1 aquatic 

 (Pisidium) species were collected, while in 

 another on a prairie slope near West Lake 

 Okoboji 3 aquatic species belonging to the 

 genera Pisidium, LymTicea and Physa were 

 collected at an altitude of more than forty 

 feet above the lake. In the latter case the 

 surrounding prairie surfaces contributed 

 no terrestrial species. 



And finally individual specimens of 

 aquatic species of mollusks may be carried 

 to uplands by aquatic birds and insects, 

 where their shells may ultimately be in- 

 cluded in terrestrial deposits. 



In order that the value of these mol- 

 luscan faunas may be properly measured 

 it is necessary that they be taken collec- 

 tively. A single terrestrial shell does not 

 make a land deposit, neither does a single 

 aquatic shell make a water deposit. In 

 water deposits aquatic shells always form 

 a conspicuous part of the fauna, even 

 though locally they may not predominate. 

 In subaerial deposits aquatic shells maj 

 occur, but they are rare and local, and the 

 dominant types are terrestrial. Strictly 

 terrestrial Pleistocene deposits are of two 

 types: buried sand dunes and the loesses. 

 Buried sand dunes are not uncommon in 

 the upper Mississippi Valley, excellent il- 

 lustrations being found near Gladstone, 

 Illinois; north of Iowa City, Iowa; at 

 Hooper and West Point, Nebraska, and at 

 other points. Neither buried nor surface 

 dunes contain shells so far as observed. 



The loesses are much more satisfactory 



