436 PHYSICAL GEOGRAPHY. 



The marking's on the valves of Pleurosigma are peculiar, and different 

 from those found upon the other naviculaeform diatoms considered. The 

 genus can be and is usually divided into two groups, distinguished by the 

 character of the markings. In the first group the valve is covered with 

 dots set all over the surface in such a way that they are in lines at equal 

 distances apart, running from the central canal to the edge of the valve. 

 But the next row starts, as it were, half a dot behind the previous one : 

 therefore its dots alternate with and come between those of the first line, 

 and so on, so that the dots are at equal distances apart all over the valve, 

 but, when traced across the valve, are in straight lines, and, when traced 

 lengthwise of the valve, are in zigzag lines. As these dots are coarse 

 and set far apart in a few species, they can then be seen to be circular, 

 but, when they approach each other closely, they appear to become, by 

 mutual compression, as would be the case if such were to occur, hexa- 

 gons. In fact, it has been one of the difficult matters to solve concerning 

 the diatoms, and one on which observers have differed for a long time, as 

 to whether the markings on certain species of Pleurosigma are circular 

 or hexagonal. Hence one species, especially named Pleurosigma angu- 

 lata, has been selected as a "test object" for lenses of moderate power. 

 But the second of the two groups into which Pleurosigma is divided has 

 its markings arranged somewhat differently. In this group the dots are 

 set in straight lines, if they be traced either across or lengthwise of the 

 valve ; that is to say, instead of being alternate, they are opposite in con- 

 tiguous rows. Among themselves the different species of Pleurosigma 

 vary somewhat : thus in many the sides run in one unbroken line from 

 end to end, being only swelled out at the centre. In P. angulata, and 

 still more so in P. quadrata, the bowing out at this point is decidedly 

 angular. In P. Balticum, a species originally found in the Baltic sea, and 

 hence the name which was given to it, but which has since been found 

 pretty much all over the world, the sides are straight and parallel until 

 near the ends, when they are curved over so that one end of the valve 

 points to the right, while the other is turned to the left. P. angulata and 

 P. quadrata belong to the group in which the dots of the markings are 

 arranged alternately, while P. Balticum has its dots set regularly and 

 opposite to each other. P. fasciola differs totally in outline from any of 

 those described. The main portion of its valve is in shape like a Navic- 



