412 DIATOM ACE^. 



loosely aggregated together as that a slight force should 

 occasion a lateral displacement of them. Still it must be 

 allowed to be very singular, that the frustules should not 

 rather separate altogether from each other, than allow so 

 great a displacement as is frequently seen to occur, and yet 

 should retain their adherence. 



A separate frustule bears much resemblance to the un- 

 dilated form of the frustule of Diatoma elongatum. Colour 

 yellowish brown. 



I am in some doubt as to the precise locality of this pro- 

 duction. It was either gathered by the Rev. D. Lands- 

 borough at Stevenston, in Ayrshire, and then probably found 

 near the sea, or (what is less likely) by myself, somewhere in 

 the neighbourhood of Cheshunt. 



72. FKAGILAKIA Lyngb. 



Char. Filaments compressed, attenuated, fragile, rarely an- 

 gular, with, in most cases, twe canals or grooves passing 

 round the centre of each frustule, indicated by the presence 

 ofpuncta on the lateral margins. 



Derivation. From frango, to break ; in allusion to the 

 brittleness of the filaments. 



The genus Fragilaria approaches very closely to that of 

 Diatoma as restricted in this work ; indeed the two genera 

 merge into each other, through Fragilaria virescens or rather 

 Diatoma virescens. The compressed character of the frond 

 is sufficient to distinguish it and all other Diatomacea from 

 the genus Meloseira. 



1. FKAGILARIA PECTIN ALIS Lyngb. 

 Plate XCV. Figs. 1. 4. 



Char. Filaments large, dark brown. Frustules in front view 

 quadrangular, two or three times broader than long, longi- 

 tudinally striated with two evident puncta on the lateral 



