NITZSCIIIA. 435 



81. GYROSIGMA Hass. 



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

 the letter S in lateral aspect ; longitudinally striated, with 

 a central depression. 



Derivation. From yvpos, a curve, and (rvy/j,a, the letter S ; 

 in allusion to the form of the frustules. 



1. GYROSIGMA HIPPOCAMPA Hass. 

 Plate GIL Fig. 11. 



Char. Frustules in front view attenuated towards each extre- 

 mity ; in side aspect lanceolate, curved, longitudinally 

 striated. 



Nav. hippocampus Ehr., Die Infus. p. 180. t. xiii. f. 11.; 

 Frustulia hippocampus Jenner, in Fl. of Tunbridge 

 Wells, p 204. 



Hob. 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. curvula of Ehrenberg.* 



82. NITZSCHIA Hass. 



Char. Frustules curved in 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 Kutzing, the construction of which term is some- 

 what objectionable, is synonymous with Gyrosigma. 



F F 2 



