Fig. 19.— The above figures represent a series of quohaugs taken January 17, 1903, 

 showing the gradation in size referred to on page 52. The clam shell and quohaug shells 

 in the lower part of the figure illustrate the method of determining the rate of growth 

 by notching the shell. The growth takes place at the edge, and the notches are left as 

 distinct marks upon the shell. 



