436 DIATOMACE^. 



front view ; straight, attenuated at either end in lateral 

 aspect. 



Bacillaria sigmoidea Nitzsch, Beitrage z. Infusorienkunde, 

 1817 (1816), p. 104. Cymhella sigmoidea Agardh, 

 Consp. Crit. Diatom. 1830, p. 11. Frustulia Nitzschii 

 Klitz., Linnaja, 1833, p. 554. t. xiv. fig. 33. Navicula 

 sigmoidea Ehr., Die Infus. p. 182. t. xiii. f. 15. t. xxi. 

 f. 11. 



A well-marked species, Avhich I conceive to be generically 

 estranged from Gyrosigma hipjiocampa by the difference in 

 the curvature of the frustule, the arrangement of the strife, 

 and the absence of the longitudinal furrow and central de- 

 pression. 



83. SPHINCTOCYSTIS Hass. 



Char. Frustules contracted in the centre; longitudinal furrow 

 and central depression absent 9 

 Derivation. From a(j)LyKros, constricted, and kvctti?, a cell. 



1. Sphinctocystis librilis Hass. 

 Plate CII. Fig. 3. 



Char. Frustules transversely striated, elongated. In front view 

 quadrangular, with straight sides and truncate extremities ; 

 in lateral aspect constricted in the centre ivith obtuse erids. 

 Navicula lihi'ilis Abhandl. d. Akademie d. Wissensch. zu 

 Berlin, 1831, p. 81. 1833, p. 267. note. Frustulia 

 quinquepunctata Kiltzing, Linnaja, 1833, p. 554. t. xiv. 

 f. 28.; Die Infus. p. 185. t. xiii. f. 21. Frustulia li- 

 brilis Jenner, in Fl. of Tunbridge AYells, p. 204. 



Hab. Cheshunt : A. H. H. Aberdeen : Dr. Dickie. War- 

 burton, Lewes, &c. : 3Ir. Jenner. 



A very curious species, which, on account of the central 

 constriction and absence of the longitudinal furrow and cen- 

 tral depression, I have ventured to make the type of a dis- 

 tinct ffenus. 



