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The Nautilus. 



Vol. XXXI. OCTOBER, 1917. No. 2 



NOTES ON THE VARIATION OF ISCHNOCHITON CONSPICUXTS CPE. 



BY E. P. CHACE. 



To the naturalist the study of variation and environment and 

 their relation to each other is always interesting and to the con- 

 chologist who studies his shells in their natural surroundings 

 as well as in the cabinet many things are revealed. It is not, 

 often, however, that variation in form may be so easily traced 

 to qualities of environment as in the following instance. 



Ischnochiton conqncuus, Cpr. is common at San Pedro and is 

 usually found on the under side of rocks in sandy tidepools. 

 In this situation they grow quite regularly, showing but slight 

 variation in form. They are active fellows and evidently sensi- 

 tive to light; for if the rocks to which they are clinging are 

 turned over, they soon glide away, always taking the shortest 

 route to the under side of the rock. 



While cleaning a lot of this species taken at San Pedro last 

 fall two specimens were noticed which differed so widely from 

 the others that they might easily have been mistaken for an- 

 other species. They were much wider and lower-arched than 

 the typical form and the posterior corners of the valves were 

 rounded off, making the lateral areas very narrow. 



Hoping to find more specimens of this odd form, a trip to 

 Point Firmen was devoted wholly to the collection of chitons, 

 with interesting results. The usual species were found in the 

 tidepools including numerous specimens of Ischnochiton con- 

 ■spicuus of the ordinary form. Ledges of soft rock beyond the 



