THE NAUTILUS. 67 



ence for at least four years looks otherwise. It is scarcely a 

 " variety " in the ordinary sense, for the intermediate terms are com- 

 pletely lacking. It is a " sport " or " mutant " in the sense of a dis- 

 continuous variant, breeding true and founding a new centre of varia- 

 tion ? To my mind the evidence, while wholly inconclusive, 

 suggests that possibility. 



T. Blaneyi and T. ruber var. index, add two extremely interesting 

 problems to New England malacology-problems which are all the 

 more interesting because further work by Mr. Blaney or some other 

 fortunately situated student, should go far to yield a solution in the 

 course of a few years. 



I may add that the feature from which var. index takes its name 

 the striking dark backward-pointing triangle on the keel of the fifth 

 valve suggests an interesting inquiry of a different nature. It ap- 

 pears to be a very wide spread and definite feature, appearing in 

 various species in various groups, now more, now less disguised by 

 conflicting color-patterns. It is by no means confimxl to the fifth 

 valve though commonest there. I find no notice of this odd Chiton 

 character in such examination of the literature as I have been able 

 to make. The fifth valve appears to have no specially significant 

 topographical relation with the internal anatomy, nor, indeed, is the 

 hollow of the keel in any of the valves occupied by any special organ 

 so far as I know. Has this mark, then, relation to some specializa- 

 tion of the complicated tegumentary system of organs? 1 



Jamaica Plain, Mass., September, 1906. 



PAPERS CITED. 



CARPENTER, PHILIP P. 



[ 7 63] Supplementary report on the present state of our knowledge with 



regard to the Mollusca of the west coast of North America. Rep. Brit. 



Assoc. Adv. Sci., 1863, pp. 517-686. 

 ['73] On the generic affinities of New England Chitons. Bull. Essex Inst., 



V. No. 9, pp. 152-154, Sep. 1873, (Reprinted in Ann. & Mag. Nat. Hist., 



4th Ser., v. XIII, pp. 119-123, 1874.) 



1 Postscript. Since the above was written I have noted that in the ''List of 

 British Marine Mollusca" published in 1902 by a committee of the Concho- 

 logical Society of Great Britain and Ireland apparently a very careful revision 

 in the course of which generic questions were well considered T. ruber and 

 T. marmorea are, indeed, united in one genus, but that genus is Tonicella. 

 How this result was reached I do not know. 



