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tlie names that have been applied to them. Tlie question 

 of what is the individual in the Diatomaceee is again raised 

 by the observance of these facts, as well as those 1 described 

 in my " Note on a point in the Habits of the Diatomaceae 

 and Desmidiacese/' read before the Boston Society of 

 Natural History, January 8, 1868, and published in their 

 'Proceedings,' vol. xi, p. 361. The specimens illustrating 

 the remarkable mode of occurrence of the two forms of 

 Gomphoneina -which I sent to Dr. Arnott unfortunately did 

 not arrive until after his death ; but, speaking of my having 

 so found them, he wrote to me in the last letter I received 

 from him as follows. I feel that I am justified in publish- 

 ing this extract as it is of such importance ; and I also 

 know, from what he wrote to me, that he himself would 

 not object to my doing so were he still living. 



" Your discovery of Gomphonema constriction and capitation 

 growing on the same stalk is interesting, if you are not deceived. 

 When a Gomphonema spore grow-s on a weed, the stalk (which 

 is merely the external mucus collected at the one end) is 

 formed by the growing frustule. It is not the stalk (or in 

 Schizonema, the tube) which produces the frustule, but the 

 frustule wdiich produces the stalk or tube. Then when the 

 frustule self-divides, several are formed, either side by side, or 

 each may project a new stalk ; but seldom with much regu- 

 larity. Now every frustule and valve arising from the same 

 spore must be precisely alike, being all formed from the original 

 frustule by repeated self-division; and as self- division merely 

 repeats the same identical form or variety, it is not easy to 

 understand how it is possible to have two varieties of form on 

 the same stipes. If there be no mistake on your part, you will 

 overturn all the present views of the productiou of new frus- 

 tules and valves. It is more easy to suppose that a frustule 

 from another stipes had become agglutinated to the stipes. But 

 as you say you have sent some in a bottle, I will examine it 

 with care when it arrives. Every spore may produce a different 

 variety, but it is not easy to understand that the same spore, 

 or stipes, can give rise to different varieties. As for the two 

 species (G. constrictum and capitatum) I have never been 

 satisfied that they are distinct, and probably G. hei'culaneum is 

 another variety." 



For my part, from the mode in which the two new valves 

 are formed within and between the two old ones, when self- 

 division takes place, I can readily understand how a great 

 variation in both outline and sculpture can occur. In this case 

 the two forms have exactly the same sculpture, and the 

 difference between them merely lies in the outline of the valve. 



