i6o NATURE'S CALENDAR 



July i^ vania, but found in great variety in all 



southern and western waters, is the river 

 mussel. These are bivalves— real fresh- 

 water clams — and they breathe, like the 

 fishes, by means of the curtain-like gills 

 hanging just beneath the two halves of 

 the shell. Some are no larger than half 

 a dollar, wdiile others grow to the size of 

 a big man's palm; some are thin, light, 

 and flattened, others heavy, robust, and 

 almost globular ; a few show curved or 

 radiating colors on a smooth, shining sur- 

 face ; more are dark and scaly, while 

 many are ornamented with ridges, or 

 knobs, or both, and nearly all have a 

 lovely mother-of-pearl interior, where 

 valuable pearls are often formed. These 

 mollusks stand on the blade edge of their 

 wedge-like forms, half-buried in the sand 

 (for they do not like real mud), and move 

 slowly about, leaving tracks like sinuous 

 furrows, by reaching out a muscular 

 " foot," gripping the ground, and drag- 

 ging the body forward. They suck in a 

 constant stream of w^ater, which brings 

 to their digestive organs minute particles 

 of floating food, animalcules, plant spores, 

 etc., and at the same time furnishes oxy- 

 gen to the blood circulating through 

 the gills. A cabinet of their shells is 

 well worth having; and ready-made col- 

 lections may often be found where musk- 

 rats and raccoons are common, for both 

 these animals delight to eat their flesh 

 and leave the shells on the bank. 



July 15 



