1892.] NEW-YORK MICROSCOPICAL SOCIETY. 5 



to one another by their corners so as to form zigzag lines, or else 

 as disconnected individuals ^vith a very striking resemblance in 

 shape to miniature skiffs or boats. If we make use of some other 

 source of supply than our Croton-water faucet, we may obtain a 

 much greater variety of forms in both the classes of objects re- 

 ferred to, though we shall not fail to notice a certain parallelism 

 in the morphology of the two families, which may easily cause us 

 to think them but one. They are, however, quite distinct, as we 

 shall see. Those forms which I have spoken of in this particular 

 case as decidedly green in color (although this is not a decisive 

 characteristic throughout the family) are known as Desmids. 

 Those which I have described as being of a brownish hue are 

 Diatoms. 



But now, if we look more closely at our desmids and diatoms, 

 we shall discover that the latter have a very rigid, hard, and al- 

 most indestructible carapace or shell, while that of the former is 

 hardly more than a stiffened membrane which is with difficulty 

 discerned at all. In the diatoms the enclosing case will prove to 

 be a solid deposit of nearly pure silica; but in the desmids it will 

 be found to be a semi-flexible film slightly infiltrated with carbon- 

 ate of lime. This is an important distinction, which forms the 

 basis for a sharp separation of the two groups of forms. As re- 

 gards the particular specimens we are supposed to have under 

 immediate inspection, however, we shall by this time have observed 

 a difference which to most minds will constitute a clearer demar- 

 cation than could be established by any other criterion; and that 

 is that our desmids are apparently quite passive, while the little 

 boat-shaped diatoms actively glide about with a seemingly well- 

 defined purpose. 



You will need to abstain from generalizing too far and too fast 

 in this connection, for all diatoms do not possess this power of 

 locomotion, since not only are some of them united to one an- 

 other, as we have seen, but many sorts are attached to plants and 

 other fixed objects, although the theory has been advanced that 

 all diatoms are, at one time or another, free and migratory. On 

 the other hand, while desmids are never permanently anchored, 

 locomotion is not a prominent characteristic of even the solitary 

 forms, because, while it is pretty general throughout the group, it 

 is too slow to attract attention, and, when observed, has not that 



