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PROCEEDINGS OF THE NATIONAL MUSEUM 



marked effect on the shells and, in cases where the pH is less than 6.0, 

 sometimes the death of the entire colony results. Many mollusks have 

 developed a chitinous periostracum that protects the shell from cor- 

 rosion. However, the abrasive action of sand particles and the attach- 

 ment by algae or egg cases of Neritina gastropods may break through 

 this outer covering and expose the calcium carbonate of the shell to 

 the acid waters. It is not uncommon for some colonies of Thiara 

 granifera to be considerably eroded at their spires. Shell-length 

 measurements, in these cases, can hardly serve as fair comparisons 

 with colonies that have suffered no loss of shell. The effect of erosion 

 may be avoided by comparing the width of shells or the height of 

 the last two or three whorls that have not been affected. 



However, it is of equal interest to learn exactly what amount of 

 reduction in length has been brought about by stream action, for 

 ecologists may wish to know not only the pH values of various streams 

 but also what effect acidity is having on the animals. A study was 

 made of a population sample of 257 specimens of Thiara gran'ifera 

 (Naujan River, Mindoro Island, Philippine Islands, U.S.N.M. 

 258890) which showed considerable erosion. It was found that adults 

 were reduced in shell length by 11.6 percent. Since this type of study 

 has not, to our knowledge, been previously used, we have gone into 

 considerable detail. 



18 MM. 



Figure 44. — I. Histogram of total shell length of sample from Mindoro colony. II. Histo- 

 gram of same shells showing height of last whorl only. 



