Mr. J. Ralfs on the Diatomacese. 105 



transverse striee ; lateral surfaces striated, contracted near the 



ends. Ehrenberg, Die Infus. p. 202. t. 20. f. 7 ; Pritch. Infus. 



p. 219. f. 180 — 182. Fragiluria striatula, Ag. Syst. p. 7 ; Kutz. 



in Linnsea 1833, p. 73 ; Grev. in Br. Fl. vol. ii. p. 403 ! F. Car- 



michaelii, Harv. Br. Alg. p. 198. Grammonema striatula, Ag. 



Consp. Diatom, p. 63. 



" Appin, Capt . Carmichael ;" Southampton, Miss Hill; co. Antrim, 

 Ireland, Mr. TV. Thompson. 



I have drawn up my description of this plant from one of Capt. 

 CarmichaeFs specimens given me by Mr. HaiTey. 



At first sight this plant bears so great a resemblance to Stria- 

 tella arcuata as to appear merely a variety of it; and without 

 taking into consideration the lateral form, it would be difficult to 

 find a good distinguishing character. 



From my specimens I was unable to ascertain whether the fila- 

 ments were attached or not ; they are fragile, and here and there 

 cohere at their angles in the same manner as Striatella arcuata. 

 The frustules are generally not so broad in proportion to their 

 length as those of Btriatella arcuata, but, like them, have nume- 

 rous series of transverse strise, which however in this plant are 

 not so strongly marked, and under a low power especially are less 

 evident than the marginal striae. 



The lateral surfaces are swollen in the centre and constricted 

 near the ends, with numerous transverse striae along their whole 

 length. 



P.S. Since the above description was written, I have gathered 

 this plant on the muddy sides of rocks in Swansea Bay. 



The filaments are more slender than in the Appin specimen, 

 of a dirty-brown colour, much curled and entangled, and but 

 slightly fragile. 



In drying it adheres much more firmly to paper than does the 

 Striatella arcuata, is less fragile, does not turn green in drying, 

 and has no gloss. 



Plate II. fig. 1. «, Tessella catena ; b, lateral view ; c, lateral view with 



the septa removed. 



Tetracyclus, n. g. 



Filaments free, fragile, four-sided, each side rounded and form- 

 ing the segment of a distinct circle ; frustules longitudinally 

 striated. 



1. T. lacustris. Frustules about twice as broad as long ; lateral sur- 

 faces with from seven to nine distinct transverse striae. 

 Llyn Prefeddyr near Barmouth, the Rev. T. Salwey ; pool near 



Dolgelley*. 



In August last this plant was sent to me by Mr. Salwey, who 



found it sparingly among Fragilaria hyemalis in the above lake. 



* I have this year met with it in two or three pools near Dolgelley. — J. R. 



