502 



SCIENCE 



[N. S. Vol. XXXVII. No. 953 



The plant remains and insects are too 

 few and too widely scattered to form a 

 satisfactory measure of conditions; the 

 mammalian fauna is but little better in 

 this respect and, moreover, consists of spe- 

 cies which are for the most part extinct, 

 and whose exact habits are not known ; but 

 the mollusks are not only most widely dis- 

 tributed in the Pleistocene deposits, but 

 belong practically without exception to 

 species now living whose habits may be 

 very accurately determined. For these 

 reasons the mollusks form the most impor- 

 tant and most significant group of Pleis- 

 tocene fossils. 



Aside from the marine species of the 

 coastal formations, which will not be here 

 considered, the Pleistocene and modern 

 species of mollusks naturally group into 

 three rather well-defined divisions accord- 

 ing to habits: 



1. Fluviatile species, inhabiting peren- 

 nial streams and lakes. These are chiefly 

 bivalves, especially larger Uiiionidw, and 

 operculate gill-bearing gasteropods. A few 

 aquatic pulmonates, like Lymncea emar- 

 ginata and Plmiorhis truncatus, also prefer 

 larger bodies of water. 



2. Pond species, inhabiting ponds, bogs 

 and borders of smaller streams, which are 

 liKely to become dry during a part of the 

 summer, and also the shallow swampy bor- 

 ders of larger bodies of water. These 

 species are chiefly aquatic pulmonates, 

 with a few smaller bivalves, especially of 

 the genus Pisidium. 



3. Terrestrial species, living among 

 plants, sometimes on the bark of trees, 

 often under leaves, sticks and stones. 

 Among them are two operculate proso- 

 branchs, closely related to aquatic species 

 of the first group, but the great majority 

 are terrestrial pulmonates. A few species, 

 such as Succinea retusa, Zonitoides nitidus, 

 Vertigo ovata and Carychium exiguum, 



naturally group with the terrestrial pul- 

 monates, but they are commonly found in 

 wet places, and sometimes live in the water 

 for a time, behaving then like the aquatic 

 pulmonates of the pond group. 



The members of the pond and terrestrial 

 groups are closely associated with plants 

 which they require for shelter and in most 

 cases for food. The terrestrial forms are 

 most abundant on forested areas, but cer- 

 tain species, such as Succinea grosvenorii, 

 Cochlicopa luhrica, Vallonia gracilicosta, 

 etc., also extend into more open territory. 



While all the terrestrial species require 

 some moisture for activity, the amount 

 which may be found at times under a leaf 

 or stick, or among plants on a dewy morn- 

 ing, or after a rain, is entirely sufficient 

 I for the purpose. This supply of moisture 

 is irregularly intermittent, and the mol- 

 lusks remain inactive while waiting for a 

 return of favorable conditions. Sometimes 

 they protect themselves by a mucous epi- 

 phragm, or they creep under leaves or 

 sticks, or burrow into the soil. Long-pro- 

 tracted dry seasons are fatal to many of 

 them. 



The fossil mollusks are distributed 

 through a variety of formations. Those of 

 the Aftonian, the Don beds, the Plorencia 

 formation and similar older Pleistocene 

 deposits, like those of the more modern 

 alluvial beds, present the fluviatile and 

 pond fades in the main, though there is 

 often a strong admixture of terrestrial 

 forms. Those of the loesses are terrestrial 

 in the main, with comparatively rare addi- 

 tions of pond types and a total absence of 

 fluviatile forms. 



Practically all of the species found in 

 the various Pleistocene formations are liv- 

 ing upon the continent to-day. Such ex- 

 ceptions as Oreohelix ioensis from the loess 

 and species of Pisidium and Lymncea re- 

 cently described from indefinite alluvium, 



