402 DIATOMACEiE. 



2. ACHNANTHES EXILIS? KutZ. 



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. 



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

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



Hab. Parasitic on Gomjjhonema geminatum in several 

 streams, near Dolgelly ; Mr. Rolfs. 



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. minutissima and Kutzing's specimen of 

 Ach. exilis. 



B. Filaments compressed, 

 A. Frustules in front view quadrangular. 



68. TETRACYCLUS Ralfs. 



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

 filament, never united by their angles, striated strongly 

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

 segment of a distinct circle. 



Derivation. From T£Tpas,four, and kvkKos, a circle. 



This genus differs only from Tessella and Striatella in the 

 frustules not cohering by the angles. 



