FRUSTULIA. 437 



84. AULACOCYSTIS Hass. 



Char. Frustules produced, longitudinally sulcated. 

 Derivation. From avXaf, a groove, and KVGTIS, a cell. 



1. AULACOCYSTIS PELLUCIDA Hass. 

 Plate CII. Fig. 8. 



Char. Frustules elongated, narroio, attenuated at each extre- 



mity. 

 Navicula? pellucida, Die Infus. p. 176. t. xiii. fig. 3. 



Frustulia pellucida Kiitz., in Linnaea, 1833, p. 543. 



t. xiii. fig. 11.; Jenner, in Fl. of Tunbridge Wells, 



p. 204. 

 Hal. Ashdown Forest, and at Shoreham, Kent ; between 



Tunbridge Wells and the High Rocks : Mr. Jenner. 



There can be no doubt of the propriety of separating this 

 species from the genus Navicula of Ehrenberg. 



85. FRUSTULIA Kutz. 



Char. Frustules elongated, quadrangular, with two longitudinal 

 grooves and two central depressions. 



Derivation. From frustulum, a small fragment. 



I must confess that I do not exactly understand the struc- 

 ture of the single species which I propose to retain in the 

 genus Frustulia. It is possible that to this genus should be 

 referred those species of Navicula which have striated frus- 

 tules ; and, if not to this genus, possibly those species should 

 be regarded as Cymbellce. 



1. FRUSTULIA? VIRIDIS Kutz. 

 Plate CII. Fig. 2. 



Char. Frustules large, elongated. In front view quadrangu- 

 lar, with straight sides and truncate extremities ; laterally 

 slightly inflated in the centre, with rounded ends. 

 FF 3 



