164 



LESLIE B. AREY AND W. J. CROZIER 



The further development of this homochromic character- 

 istic is conditioned by, or goes together with, the erosion of the 

 shell valves. This becomes noticeable at the age of five to six 

 years, commonly, and may be exhibited in a non-subjective 

 manner, as follows: 



If the length of the tegumentum along the middorsal line of 

 the fourth valve be measured for a series of chitons from any one 

 locality, and plotted against the length (or the estimated age) of 

 the corresponding individuals, a graph is obtained (fig. 2) show- 



14 



o 

 ^ 6 



6 — 



c7)4 

 c 



5^ 



2. - 



125456763)0 cms. 

 Lenc|fh of indiu/dual. 



Fig. 2 Showing the relation between length of the fourth valve, in milli- 

 meters, and the total length of the individual in centimeters, for chitons of a range 

 of sizes. (Marshall Island, south side. May 9, 1918.) See text and fig. 3; a, 

 point at which 'beak' begins to project; /S, 'erosion point,' estimated from the 

 curve; y, range of individuals in which (on inspection) erosion was judged to be 

 just beginning. 



ing one point (a) of sharp bending, at an early age, followed at a 

 more advanced age by a slower change in curvature (/3), initiating 

 a region of less decided correlation between valve length and 

 length of individual. These changes in the course of the graph 

 originate in the following way: In very young chitons the beak 

 ('umbo') on the fourth valve is quite undeveloped, and does not 



