106 Mr. J. Ralfs on the Diatomacese. 



I afterwards gathered a specimen of the same Fragilaria near 

 Dolgelley, with which a few of the frustules were intermixed. 



I was at first inchned to refer this plant to Ehrenberg's genus 

 TesseUa, but more minute examination comdnced me that it could 

 not be placed there. The frustules are striated as in Striatella 

 and TesseUa, but it differs from the former genus in not being 

 attached, and from both, as the frustules do not cohere at the 

 angles forming a chain. In the cm-ious form of the filament it 

 differs equally fr'om these genera and all the other Diatomacea, 

 except perhaps Tabellaria, in which the inflated canal produces a 

 distant resemblance. 



Filaments attenuated, fom--sided ; the sides are all rounded : 

 hence, when either of the sides is examined by the microscope, 

 portions of two others are also seen like a border, and the outlines 

 of that under examination appear like two lines running down the 

 filament. But the opposite sides only are alike. Two of them 

 are in fact the margins of the filament, and may be distinguished 

 fi-om the other two by a slight contraction of the filament at the 

 junction of the frustules on the front and back, by the absence of 

 puncta at that part, and by their striae being more distinct. A^Tien 

 the front and back are viewed imder the microscope, there is no 

 contraction of the filament at the junction of the frustules, and 

 the ends of the lateral strife exhibit puncta along the margin. 



In this view larger portions of the older surfaces are seen, and 

 hence the filament appears to have a broader border, in which the 

 strise are more distinct than in the centre. 



The endochrome is of a dark green colom-, and is often col- 

 lected into an irregular spot. 



The lateral sm-faces resemble in outline the flower of a Veronica, 

 or the quatrefoil of a gothic ivindow ; their length and breadth in 

 the smaller frustules are nearly equal, but in the larger ones they 

 are 1|^ as long as broad. They have about seven rather distant, 

 w^ell-marked transverse stride. 



Plate II. fig. 2. a, I'etraajclus lacustris ; b, marginal view; c, lateral 

 views; d, lateral views, showing its internal plates. 



Fragilaria, Lynyb. {Eh.) 



Filaments free, fragile ; frustules rectangular, without striae on the 

 central portion. 



This genus is nearly allied to Diatoma ; the chief distinction 

 between them is, that in Diatoma the frustules cohere at the angles, 

 and in Fragilaria do not. E\'en this cUstinction is not always 

 present in one species of Fragilaria, which seems therefore inter- 

 mediate. 



The filaments are free, generally attenuated and fragile ; the 



