NITZSCHTA. 



435 



81. GYROSIGMA Hass. 



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

 the letter S in lateral aspect ; longitudinally striated, with 

 a central depression. 

 Derivation. From ^vpos, a curve, and acyfia, the letter S ; 

 in allusion to the form of the frustules. 



1. Gykosigma hippocampa Hass. 

 Plate CII. Fig. 11. 

 Char. Frustules in front view attenuated toioards each extre- 

 mity ; in side aspect lanceolate, curved, longitudinally 

 striated. 

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

 Frustulia hippoca}7ipus Jenner, in Fl. of Tunbridge 

 Wells, p 204. 

 Hab. Cheshunt: A. H. H. Olford and Shoreham, Kent : 

 Mr. Jenner. 



In the same genus with this species should be placed the 

 Navicida sigma, N. scalprum, and N. curvida of Ehrenberg.* 



82. NITZSCHIA Hass. 

 Char. Frustules curved in front vieio in the form of the letter 

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

 out a longitudinal furroto 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 frustulcs. 



1. NiTZSCHIA ELONGATA Hass. 



Plate CII. Fig. 12. 



Char. Frustulcs linear, lengthened, xoith truncate extremities in 



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

 what objectionable, is synonymous with Gyrosigma. 



FF 2 



