68 Yearns — Fresh Water Mussel Shells. 



The small proportion of limj' to animal matter is con?pic- 

 uoiislj^ exhibited in tlie tendencj'^ of tlie thin Anochnta shells to 

 crack througli the shrinking of tlie periostracum, not infre- 

 quentl}' fracturing a specimen beyond repair. 



These proportions of lime and animal matter (to use simply 

 language), are apparently reversed wlion tlie shells of Unio are 

 compared with those of Anodontn. 



Dr. Philip Carpenter, ^Yriting 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 innnense development of this group of 

 heavy shells. The North American T'nios may be regarded as 

 so many Avater-filters absorlnng the lime from the water, and 

 preserA^ing it from re-absorption l)y their strong, hornj- 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 

 probablj', 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 n/gnea we find a companion in its extra- 

 ordinary- range of distribution in Margnritavn margaritifera. 

 This wide distribution is not only geograpiiic in the ordinary' 

 sense but hypsometric also, and this companionship includes 

 the West American forms oljserved 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 soniewliat fanciful. 



