278 



Bacillariece 



Family 1. RHIZOSOLENIACE^I. 



The cells are elongated, more or less cylindrical, and often form 

 chains. In cross-section (or in the valve-view) they are either 

 circular or broadly elliptical. The extremi- 

 ties of the cells are attenuated and either 

 symmetrical or asymmetrical. In the latter 

 case there is a calyptra or hood surmounted 

 by a sharp spine or a long needle-like seta. 

 This attenuation of the extremities of the 

 cells is really a more or less excentric, coni- 

 cal projection from the valve-faces. The 

 girdle exhibits numerous rings of large 

 rhomboidal scales, and the frustules are 

 sometimes imperfectly siliceous. The chro- 

 matophores consist of numerous small plates, 

 generally somewhat elongated. 



Genus Rhizosolenia Ehrenb., 1858 ; em. 

 Peragallo, 1892. The frustules are very 

 elongated and subcylindrical. The valves 

 are asymmetrical, terminating in a hood 

 which is furnished with a spine or a long 

 seta. The spines or bristles are excentric, 

 but are disposed parallel to the long axis of 

 the cell. The girdle region of the cell con- 

 sists of rings of scales, which are more or less 

 imbricate. The chromatophores are small, 

 rounded or elongated plates. The auxo- 

 spores are formed without conjugation. 

 Hypnospores are sometimes produced, one 

 or two in each mother-cell. 



Fig. 128. Ehizoso- 

 lenia longiseta Zach. 

 (After Schroder, x 544.) 

 A, frustule showing rest- 

 ing spore; B, germina- 

 tion of resting spore. 



Most of the species of this genus are marine, 

 but two are known to occur in the freshwater 

 plankton of the British Islands. R. longiseta Zach. 

 (fig. 128) occurs in the plankton of Loch Shin and 

 Loch a Gharbh Bhaid Mhoir, Sutherland, and 

 R. eriensis H. Smith also occurs in Loch Shin. 



Genus Cylindrotheca Rabenh., 1859. The frustules are 

 symmetrical and spindle-shaped, with the apices much attenuated. 



