THE BRITISH DIATOMACE^. 



Genus 30. MERIDION, Ag. 



Frustules cuneate, united into a filament, at first attached, at length 

 free, and spiral ; valves elongate, convex, furnished with transverse 

 costae, striated. 



A close analogy exists between this genus and Licmophora. In 

 both the frustules are cuneate, and adhere by their contiguous valves ; 

 consequently, during the process of self-division, a filament of a cir- 

 cular form becomes developed. In Licmophora the circular arrange- 

 ment is speedily interrupted by the dichotomy of the stipes to which 

 the united frustules are permanently attached ; but in Meridion, the 

 attachment being confined to the earliest stages of growth, and the 

 frustules speedily becoming free, nothing interrupts the progress of 

 the circular development save the imperfect adherence of the valves, 

 or the occurrence of extraneous force. It is therefore not unusual to 

 find the circuit completed, or the filament still farther extended into 

 a coil of one or two superimposed circles, though, from the fragility 

 of the band, it is ordinarily broken up into semicircles, or still smaller 

 fragments, many consisting of not more than three or four frustules. 



When first developed, the single or double frustules of Meridion 

 bear a close resemblance to those of a Gomphonema ; but the absence 

 of a stipes serves to distinguish them from the stalked forms of that 

 genus, and the peculiar markings of the valves offer sufficient means 

 of discrimination in all other cases. 



As we now enter upon the filamentous species, it may be necessary 



