68 Stearns Fresh Water Mussel Shells. 



The small proportion of limy to animal matter is conspic 

 uously exhibited in the tendency of the thin Anodonta shells to 

 crack through the shrinking of the periostracum , not infre 

 quently fracturing a specimen beyond repair. 



These proportions of lime and animal matter (to use simple 

 language), are apparently reversed when the shells of Unio are 

 compared with those of Anodotita. 



Dr. Philip Carpenter, writing nearly fifty years ago, with the 

 Fresh-water Mussels of the Mississippi drainage in mind, re 

 marked : '' In no other known portion of the earth is there so 

 large an area covered with soluble limestone. The water of the 

 rivers being saturated with this would be unfit for many of its 

 uses, were it not for the immense development of this group of 

 heavy shells. The North American Unios may be regarded as 

 so many water-filters absorbing the lime from the water, and 

 preserving it from re-absorption by their strong, horny skins."* 



These few lines suggest the following questions: 



First. Is the nearly absolute lack or absence of the Unio form 

 in the drainage basins of the Columbia and Sacramento rivers 

 probably, or measurably, due to a smaller proportion of lime in 

 the waters of said basins, as compared with the Mississippi 

 waters? 



Second. Is there a corresponding discrepancy or absence of 

 the Anodonta form in the Mississippi basin? 



To the latter query only one answer is possible. 



Turning back to the A. cygnea group and the consideration of 

 the wide dispersion of cygnea we find a companion in its extra 

 ordinary range of distribution in Margaritas a margaritifera. 

 This wide distribution is not only geographic in the ordinary 

 sense but hypsometric "also, and this companionship includes 

 the West American forms observed in the Columbia and Sacra 

 mento basins. 



While inhabiting the same waters, though not as heavy as 

 many of the Unios of a corresponding size, the proportion of 

 limy to animal matter is much greater than in the thin-shelled 

 Anodons herein mentioned. 



The coincident distribution of these two forms suggests some 

 thing more than an unrelated and isolated fact. 



* The presumed unfitness of the water of the rivers for many of its uses, being neu 

 tralized by the Unios, etc., may be regarded as somewhat fanciful. 



