NATURAL HISTORY OF THE DIATOMACEAE. 435 



There is a genus very closely allied to Pinnularia which has the same 

 general form, except that in many of the species the sides slope off 

 straight towards the somewhat acute ends, so that the whole valve is 

 quadrangular in form. On account of the thickened portion at the centre 

 of the valve being widened out so as to extend almost or quite across the 

 valve as a band, and thus, with the median blank space, form a cross, it has 

 received the name of Stauroucis. The markings on the valve, however, 

 are not those found on Pinnularia, but consist of minute depressions, 

 or dots set in lines, which run usually somewhat sloping from the middle 

 portion to the edge. These rows of dots are usually known as "striae," 

 and are often extremely fine, so much so that in some species in which 

 they occur they are very difficult to demonstrate, and hence the diatom 

 becomes a very good "test-object." In Navicn la, the genus which has 

 by far the largest number of species, inhabiting fresh, salt, and brackish 

 water, we have Stauroneis without the central cross bar, but merely the 

 blank longitudinal space found in Pinnularia. 



The variation in outline and in other respects, among the several hun- 

 dred species which have been grouped together under the generic name 

 of Navicula, is very great, so much so, in fact, that it would seem reason- 

 able to believe that several genera must have been unconsciously fused 

 together. And such is the opinion of the present writer, in which he 

 rather agrees with some of the older writers on the diatomaceae. 



There is a genus which at one time was included in Navicula, but 

 which has of late years been separated therefrom, and is known as Pleu- 

 rosigma. It looks like a Navicula which has been twisted so as to bend 

 the two opposite sides of the valve in different directions. Hence it has 

 somewhat the form of an S, as its name indicates. It has a central canal 

 like the other naviculasform diatoms ; but the blank space through which 

 it runs is very narrow. The central expansion is generally present ; but 

 the terminal swellings are not so evident. On this account, although in 

 Pinnularia and other genera, in which they are pronounced, the terminal 

 expansions of the blank, thickened portion have been called "terminal 

 nodules;" in Plcurosigma, where they are not so apparent, that name 

 has been applied to the swelled ends of the central canal, an example of 

 the unscientific manner in which the diatomaceae have been treated by 

 many who have written about them. 



