TETRACYCLUS. 403 



1. TETRACYCLUS LACUSTRIS JRalfs. 

 Plate XCIII. Figs. 8, 9. 



Char. Frustules about twice as long as broad. Lateral sur- 

 faces with from seven to nine distinct transverse strict ; in 

 end \iewfour-lobed. 



Hob. Llyn Prefeddyr, near Barmouth : Rev. T. Salwey. 

 Pools near Dolgelly : Mr. Ralfs. 



In consequence of the four-lobed form of the frustules 

 composing a filament, and which arises from the inflation of 

 the central part of each frustule, it happens that when ex- 

 amined by the microscope that the centre of one side or lobe 

 is seen together with a portion of two other lobes, one on each 

 side, the boundaries of the lobe in view being indicated by two 

 lines running down the length of the filament. But as the 

 lobes are not all of equal size, those only being so which are 

 opposite to each other in the filament, it follows that the 

 quantity of the other two lobes seen varies according to the 

 side of the filament which is uppermost. Now, the longer sides 

 or lobes are those which form the margins of the filaments, and 

 those which form the centre the shorter, the filament rests of 

 course usually upon one of the shorter lobes : from this it re- 

 sults that in the filaments in their usual position, that a con- 

 siderable portion of the lateral lobes is seen, but in the cases in 

 which they rest upon their longer or marginal lobes a very 

 small portion only of the lateral lobes is in view. The smaller 

 and shorter lobes correspond to the back and front of the fila- 

 ment, and the larger and longer to the margins, which may be 

 distinguished in addition to their breadth by the presence of 

 stria?, more strongly marked than they are in the centre of 

 the filament : the terminations of the frustules in this view 

 are also punctated, although not contracted. In the marginal 

 view, that is with the margin of the filament uppermost, but 

 a very slight portion of the back and front lobes is seen, the 

 striae are most strongly marked in the centre, and describe 

 quadrangular spaces, the frustules are contracted also at their 

 points of junction where they present no puncta. 



DD 2 



