BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [12] 



The presence of carbonic acid (carbon dioxide) released by the decay of 

 vegetation and taken up by the water is not only unfavorable to mollus- 

 can life directly when present in quantity, but indirectly, by causing 

 erosion of exposed parts of the shell and rendering necessary an ex- 

 ceptional secretion of shell substance to repair these damages, is weak- 

 ening to the animals and very injurious to the shells, viewed as material 

 for study. 



The range of temperature borne by fresh- water mollusks is very 

 great. Some species of Fisidium, Sphcerium, and Limncea are found at 

 great altitudes (6,000-14,000 feet) in pools where the temperature sel- 

 dom rises much above the melting snow from which they spring. On 

 the other hand species of the same genera as well as Planorbis and gas- 

 tropods belonging to the Amnicolidce occur in the waters of thermal 

 springs which have a high temperature. Most of the fresh- water mol- 

 lusks appear to enjoy a certain amount of sunlight, or at least are not 

 repelled by it, though as Dr. Lea has shown, some of the Naiades (and 

 possibly all of them), though without organs of sight, are distinctly 

 sensible to the influence of light. On the other hand a certain number 

 of species, mostly of small size, frequent subterranean waters, deep- 

 wells, or the lower depths of great lakes. 



The presence of impalpable mud suspended in the water is as ob- 

 noxious to gill-bearing mollusks as clouds of dust are to air-breathing 

 animals, and for the same reason. Salts of borax, magnesia, soda, and 

 potash are injurious, and, when abundant, are fatal to molluscan life, 

 although when the change is brought about with extreme slowness 

 some hardy species may survive in the presence of salt enough to ren- 

 der the water perceptibly brackish. The effect of the change is gen- 

 erally apparent in a tendency of the shells to become ribbed, dwarfed, 

 abnormally thickened, or distorted under such conditions. Conversely, 

 marine species, if the change be sufficiently gradual, may survive in 

 water which has become nearly or quite fresh, and thus present the 

 naturalist with such anomalies as the fresh- water arks, cockles, and 

 teredos of India and the river limpets [Actncea) of Borneo. 



As lakes become saline the species which can not migrate die, but 

 those capable of doing so are apt to retreat to the streams feeding such 

 lakes, and finally to the springs which feed the streams. So that in 

 the case of the Pleistocene Lake Bonneville, of which the Great Salt 

 Lake of Utah is a remnant, the mollusks, long thought to be extinct, 

 which lie fossil in myriads on the ancient shore lines, have one by one 

 been discovered living in the few springs which remain in that arid 

 region, until the discovery of Tryofiia, in 1891, by Dr. Merriam, made 

 the list complete. In mountain pools, wet meadows, ditches, and 

 ponds are found the small bivalves of the genera Fisidium and Splice- 

 Hum; in larger ponds, canals, and slow-moving waters Anodonta flour- 

 ishes, while Unio and the larger Naiades seem to prefer streams, often 

 lying in full view on a hard rocky bottom or slowly pushing themselves 

 about on the mud or sand. Marshes give shelter to the operculate 



