THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY, 



SUPPLEMENT TO VOL. XIIL JUNE 1844. 



LVI. — On the British species 0/ Achnantlies. 

 By John Ralfs, Esq., Penzance*. 



[With a Plate.] 

 AcHNANTHES, Bory [Ag.). 

 Frond stipitate, standard-shaped, generally composed of few frus- 

 tules, which are longer than broad, curved, and have a punc- 

 tum at the centre of the inferior margin. 



Frond generally composed of few frustules, and then flag-like, 

 but sometimes elongated into a filament, as in the Ach, brevipes 7 

 of Kutzingf. It is attached by one of the lower angles of the 

 basal frustule ; in some species the stipes is elongated, and in 

 others very short. 



The frustules are longer than broad, and curved or bent ; the 

 upper margin is convex and the lower one concave, the latter 

 having in the middle a minute pellucid dot or punctum. In 

 some species the lateral portions are turgid, the central portion 

 looking like a band between them ; in others they are flat, and do 

 not enter into the front view. The superior lateral surface difi^ers 

 from the lower one in the absence of the central transverse pel- 

 lucid line which is present in the lower, and by its termination 

 forms the punctum seen in the front view. 



The mode of growth in this genus resembles that of Isthmia, 

 except that the frustules finally separate without cohering at their 

 angles. 



Achnanthes difi'ers from the other Diatomacece except Striatella 

 by its stipitate flag-like fronds, and from Striatella it may be 

 known by the absence of internal siliceous plates ; and even a 

 solitary frustule of this genus, separated from the stipes, may be 

 distinguished from that of any other by its curved form and the 

 punctum in the middle of the lower margin. 



In determining the British species I have been much assisted 

 by the Rev. M. J. Berkeley and Mr. Borrer ; to the latter I am 

 indebted for the opportunity of examining the specimens in Kut- 

 zing's ^ Alg. aq. dul.' 



* Read before the Botanical Society of Edinburgh, Feb. 8, 1844. 

 t I have received from Mr. Harvey a specimen of this or an allied form, 

 gathered at the Cape of Good Hope. 



Ann.^Mag,N.Hist, Vol^iu Suppl 2K 



