436 DIATOMACE^. 



front view ; straight, attenuated at either end in lateral- 

 aspect. 



Bacillaria sigmoidea Nitzsch, Beitrage z. Infusorienkunde, 

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

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

 Kiitz., Linnaea, 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, which I conceive to be generically 

 estranged from Gyrosigma hippocampa by the difference in 

 the curvature of the frustule, the arrangement of the striae, 

 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 ? 

 Derivation. From O-^L^KTOS, constricted, and /evens, a cell. 



1. SPHINCTOCYSTIS LIBKILIS 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 with obtuse ends. 



Navicula librilis Abhandl. d. Akademie d. Wissensch. zu 

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

 quinquepunctata Kiitzing, Linnsea, 1833, p. 554. t. xiv. 

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

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



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

 burton, Lewes, &c. : Mr. 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 genus. 



