402 DIATOMACE^. 



2. ACHNANTHES EXILIS ? KtltZ. 



Plate C. Fig. 5. 



Char. Frustules slender^ from one to nine in each jrond. 



Lateral surfaces subacute. Striae indistinct or wanting. 



Stipes longer than the frustule. 

 Klitz. Syn. Diatom, p. 578. f. 53. ; Ehr. Infus. p. 228. 



pi. XX f. 5. ; Ralfs, in Annals, vol. xiii. pi. xiv. fig. 12. 

 Hah. Parasitic on Gomphonema geminatum in several 



streams, near Dolgelly : 3Ir. Ralfs. 



From Achnanthes minutissima this species differs in its 

 elongated stipes, greater number of frustules in each frond, 

 and in its more acute lateral surfaces. 



Although I have compared our plant with Kutzing's 

 Achnanthes exilis I am not certain that it is the same species. 

 It agrees with Kutzing's specimen in its crowded habit and 

 elongated stipes, but its frustules are much smaller, and 

 its lateral surfaces less acute, in both which respects it is inter- 

 mediate between Ach. mi7mtissima and Kutzing's specimen of 

 Ach. exilis, 



B. Filaments compressed. 

 A. Frustules in front view quadrangular^ 



68. TETRACYCLVS Ralfs. 



Char. Frustules coherijig so as to form a fragile, attenuated 

 filament, never united by their angles, striated strongly 

 laterally ; foui-- sided, each side rounded, and forming the 

 segment of a distinct circle. 



Derivation. From rsrpas, four, and kvkXos, a circle. 



This genus differs only from Tessella and Striatella in the 

 frustules not cohering by the angles. 



