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THE AMERICAN NATURALIST [Vol. LIIL 



in many cases one of the necessary preliminaries to the migration 

 from land to water, and that the aquatic angiosperms thus in- 

 clude, by a process of sifting,^ those plants whose terrestrial an- 

 cestors were endowed with a strong tendency towards hetero- 

 phylly. 



Agnes Arber 



Newnham College, 



COALESCENCE OF THE SHELL-PLATES IN CHITON* 



Chitons are peculiar in the fact that the molluscan shell is 

 here represented by a series of eight distinct dorsal plates, which 

 in different genera overlap and articulate with one another to 

 varying degrees. The full number, 8, seems, however, to be 

 invariably present. "While examining recently a series of some- 

 what over 2,100 individuals of Chiton tuherculatus L., I came 

 upon two cases, and two only, exhibiting any irregularity with 

 respect to the number of the shell-plates. These were specimens, 

 a male and a female, found near together on the beach at Cross 

 Bay, Bermuda, in which plates 7 and 8 had in each instance 

 almost completely fused (Figs. 2-6), so that each of these ani- 

 mals seemed at first sight to have but 7 plates; since no rec- 

 ords seem previously to have been made of such occurrences, 

 they are here figured and described. 



In the two abnormal chitons the fused terminal plates were 

 of similar external appearance, but in individual A, the female, 

 the coalescence of plates 7 and 8 was somewhat less complete 

 than in individual B, the male, as shown by the form of the 

 inner surfaces of the compound plates. It is perhaps accidental 

 that in both cases fusion of the respective plates is somewhat 

 assymetrical, being more complete on the right side. As seen 

 in Fig. 4, the muscular intersegmentum, which ordinarily re- 

 ceives the insertion plates of the eighth valve, is represented by 

 a relatively smf^ll tongue of tissue. 



This great sifting experim 

 :et a glimpse at Nature in 

 ibutions from the Bermuda ] 



