NITZSCIIIA. 435 



81. GYRO SIGMA Hass. 



Char. Frustules straight in front view ; curved in the form of 

 the letter S in lateral aspect ; lonfjitudinalhj striated, loith 

 a central dejiression, 

 Deinvation. From <yvpos, a curve, and ai'yiJba, the letter S ; 

 in allusion to the form of the frustules. 



1. Gyrosigma hippocampa Hass. 

 Plate CII. Fig. 11. 



Char, Frustules in front view attenuated towards each extre- 

 mity ; in side aspect lanceolate, curved, longitudinally 

 striated. 

 Nav. hip-pocam-pus Ehr., Die Infus. p. 180. t. xiii. f. 11.; 

 Frustulia hippocaminis Jenner, in Fl. of Tunbridge 

 Wells, p 204. 



Hah. Cheshunt : A. H. H Olford and Shoreham, Kent : ^ 

 Mr. Jenner. 



In the same genus with this species should be placed the 

 Navicula sigma, N. scalprum, and N. ciirvula of Eln'enberg.* 



82. NITZSCHIA Hass. 



Char, Frustules, cwrye^ m front view in the form of the letter 

 S; in lateral aspect straight, transversely striated, and with- 

 out a longitudinal furrow and central depression. 

 Derivation. Named in memory of M. Nitzsch. 



The single species upon which this genus is founded 

 accords with Exilaria in all its characters, save in the seg- 

 moidal curvatures of the frustules. 



1. NiTZSCHIA ELONGATA HasS, 



Plate CII. Fig. 12. 



Char. Frustules linear, lengthened, with truncate extremities in 



* The Sigmatella of Kiitzing, the construction of which term is some- 

 what objectionable, is synonymous with Gyrosigma. 



F F 2 



