NATURAL HISTORY OF THE DIATOMACE^. 437 



tila, being almost oval, but pointed at each end, and with a central canal 

 running down the middle, just like a Navicula; in fact, it may be said to 

 be almost shuttle-shaped; but from each extremity projects a long, horn- 

 like portion, into which the central canal is continued, and which is 

 curved one to the right, the other to the left, thus completing the sigmoid 

 form necessary to constitute a member of this genus. Some species of 

 Pleurosigma have markings of such extreme fineness that it becomes 

 very difficult to see them unless the microscope employed be of the best 

 quality. A few species are found in fresh water, but for the most part 

 they are inhabitants of the brackish water of swamps and similar localities. 



Some species of diatoms present us with examples of a peculiar struc- 

 ture not found in all. Thus, when we look at certain species of Stauro- 

 neis from a front view, we see at the ends and just below the terminal 

 nodules, as a part of the valve, and just where it joins the connecting 

 membrane or zone, a stout siliceous structure projecting a short distance 

 into the cavity of the frustule like a shelf, but more so at the ends than 

 at the sides, where it becomes so shallow as to be hardly apparent. 

 When we look down upon the valve, or view the diatom on what is known 

 as the side, we see that this projecting portion forms a ring all around 

 the cavity, widest at the ends and narrowest at the centre, where it is 

 hardly, or in some species not at all, perceptible. This has been called 

 the "septum." In Triceratium, we find it appearing only as slight project- 

 ing shelves at the corners. Some genera have it very strongly developed, 

 and projecting very far into the cavity, so that the two septa divide it 

 into three distinct portions. Some genera have more than two septa, 

 and in such cases they are not fused with and form part of the valves, 

 but are attached to separate rings which lie between the edges of the 

 valves and the connecting membrane or zone. 



Septa are very marked characters in a genus named Grammatophora, 

 which is found in chains of many frustules, united corner to corner, and 

 attached to algse in the ocean on almost all sea shores. The valves are 

 shaped somewhat like a Pitmularia, but have the stri^, as the rows of 

 markings have been called, running straight across, and extending quite 

 up to the ends. The septa are four in number, and extend almost across 

 the cavity of the frustule, leaving only a small opening of communica- 

 tion at the centre. Besides, although some species have them straight. 



