NATURAL HISTORY OF THE DIATOMACEvE. 433 



As has been said, the species of Biddulphia vary very greatly among 

 themselves, until we have them approaching the boat-like form in outline, 

 and in this way they are connected with the next group of diatoms, which 

 we now come to consider. These are all more or less quadrangular in 

 outline, or, rather, we might describe them as elongate, with more or less 

 parallel sides, and having their ends acute or rounded. The genera 

 belonging to this group are very numerous, but we shall describe only a 

 few of them. It is a remarkable fact, that, almost with the exception of 

 some of the Melosirce and allied cylindrical forms, the discoid, two-, three-, 

 four-, five-, and six-cornered genera are confined to salt water ; but the 

 boat- and stick-like forms are found in both fresh and salt water, so that 

 in our streams and ponds we find the "naviculaeform" and "bacillar'' 

 genera, as they are called, almost unmixed with circular forms. A few 

 species of Melosircs, and an allied genus called Cyclotella, are found 

 occasionally intermixed with the boat-like ones, and rarely alone in lakes 

 and fresh- water streams. So we already see that, by examining the 

 species of diatoms, we can say with considerable certainty whether a 

 piece of water be fresh or salt, and, if found in a fossil condition, whether 

 the earth, of which they make up a part or the whole, was thrown down 

 from a now extinct ocean, lake, or river. Our knowledge of the habits of 

 the diatomacese is hardly complete enough as yet for us to tell exactly the 

 character of lake, river, or ocean in which the diatoms grow ; but already 

 we have learned that certain forms are found on mountain tops, others in 

 swift streams, and so on. 



Taking Pimmlaria as the first type of the naviculaeform, or boat- 

 shaped diatoms, we find it to be, of course, made up of two siliceous 

 valves and a connecting membrane. The valves are boat-shaped in 

 outline, sometimes with the sides parallel, and the ends pointed or 

 rounded off. Frequently the sides are convex or bowed outwards, bent 

 inwards, or undulate ; but all of the various species, and they are very 

 numerous, preserve the general characteristic boat-like form. The valves 

 in this genus are commonly very convex, so that when looked at on a 

 front view or endwise, the edges are distinctly rounded off. Running 

 down the middle portion of the valve from one end to the other is a 

 blank space, which, at each end and at the centre, expands into round 

 nodules projecting into the cavity of the frustule, as likewise does the 

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