520 



SCIENCE 



[N. S. Vol. XLIX. No. 1274 



in Lac Leman at depths of 25 to 250 meters.- 

 In the Swedish lake Vatter, mollusks, includ- 

 ing lymnseas have been dredged from depths 

 down to 120 meters.' In America no such 

 depths have yielded mollusks in our fresh- 

 water lakes, the deepest water from which 

 ISTnnseas have been found being in Lake Mich- 

 igan where several species were obtained at 

 High Island Harbor at a depth of 10 meters.'' 

 It is possible that some of the deep western 

 lakes, as Lake Pend d'Orielle, Idaho, examined 

 by the writer some time ago, may contain a 

 deep-water fauna. 



The species cited as an example of abysmal 

 lymnseas, Acella haldemani (Desh.) Binney, 

 is really a shallow-water, swamp-loving species, 

 when adult, in the fall, living at or near 

 the surface attached to vegetation in water 

 less than five feet deep. In summer (July) 

 the young may be found in water not exceed- 

 ing six feet deep, among such plants as 

 Potamogeton.^ 



The present center of distribution of Amer- 

 ican lymnseas is the Canadian faunal regioii, 

 where upwards of 50 species and races live. 

 ISTorth and south and east and west of this 

 area there is a more or less rapid decrease 

 in number of species. It is in this area that 

 we find the greatest variation in the ecological 

 relations of the group. This is due in part to 

 the effect on the topography made by the 

 great ice sheet which swept over the territory 

 during the Pleistocene Period, and left upon 

 its retreat the largest number of ponds and 

 small lakes known in any part of the world. 

 As typical pond and lake animals, the lymnseas 

 have reacted favorably to this profusion of 

 small bodies of water and a large and varied 

 .fauna has resulted. This is also true of other 



2 Forel, Bull. Soc. Vaudoise des Sci. Nat., X., p. 

 217; XIII., p. 1 (1869, 1874). 



s Internationale Revue der gesamten Hydro- 

 hiologie und HydrograpMe, Band 7, Heft 2-3, pp. 

 146-204, 1915. 



4 "Walker, Nautilus, IX., pp. 3-5, 1895. 



« For the life history and ecology of this species 

 see Baker, Nautilus, XXX., pp. 135-138; Tech. 

 Pub. No. 9, N. Y. State College of Forestry at 

 Syracuse University, 1918. 



groups of fresh-water mollusks especially the 

 related fresh-water pulmonates, Planorhis and 

 Physa.^ Ecologically, our lymnEeid fauna may 

 be divided into about five types : 



1. The open shore lahe type, where the en- 

 vironment is an open shore of a lake exposed 

 to the full force of the winds and waves. 

 Here such lymnseas as Lymnosa stagnalis lil- 

 liance Baker, Galba catascopium Say, O. 

 emarginata Say, and O. nasoni Baker are 

 common and typical of such a habitat. 



2. The quiet hay or pond type, where the 

 environment is protected from the force of the 

 waves and wind by barriers of one kind or 

 another. The water is shallow and there is 

 usually an abundance of vegetation, such as 

 Scirpus, Potamogeton, Castalia, Nymphwa, 

 Typha and filamentous algse which provide 

 much of the food of these snails. Such species 

 as Acella haldemani (Desh.) Binn., Pseudo- 

 succinea columella (Say), Bulimnea megasoma 

 (Say), and Lymncea stagnalis Linne are typ- 

 ical of such a habitat. 



3. The marsh type, where the water is shal- 

 low, seldom more than three or four feet deep, 

 and where there is an abundance of swamp 

 vegetation such as Typha, Pontederia, Decodon 

 and a few Nymphwa. The bottom is usually 

 of mud or accumulated vegetable debris. 

 Such species as Galha palustris (Miill.), G. 

 ohrussa (Say), G. refiexa (Say), and G. elodes 

 (Say) are characteristic of this kind of a 

 habitat. 



4. The mud-flat type. This type of habitat 

 may border a swamp, pond or river, where the 

 water is quiet and where an area of wet mud 

 is left just above the water line. Here small 

 species of the subgenus Simpsonia are at home 

 and we find such species as Galha parva 

 (Lea), G. dalli (Baker), G. umhilicaia 

 (Adams), and some of the small varieties of 

 G. olrussa (Say) living by thousands, simu- 

 lating the marine Littorinas in their ecolog- 

 ical relations. 



5. The intermittent pool or stream. This is 

 a type of habitat found in all parts of the 



6 Baker, "Monograph Lymn. N. A.," pp. 52-67, 

 1911. 



