260 



RHOICOSIGMA. 



GENUS 23. RHOICOSIGMA GRUN, 1867 

 Emend H.P. 1891. 



Frustules more or less torsive, 

 achnanthiform. Valves sigmoid, 

 dissimilar ; the upper valve deli- 

 cate, convex, with raphe straight 

 or gently flexed ; the lower more 

 robust, concave, with raphe very 

 sigmoid and more or less carinate. 

 Stria? decussate or rectangular. 



The genus Rhoicosigma includes 

 about 15 species, of which only the 

 two following belong to our latitude. 

 Prof. Cleve does not recognise this 

 genus, but places the species amongst 

 the Pleurosigma 



Fig. 51. 

 Rhoicosigma Oceanicum 1 1. P. [after H, 



Perogallo). 



{ 



ANALYSIS OF SPECIES. 



Striae decussate, frustule arcuate, not achnanthiform . 

 Stria; rectangular, frustule rather flexed, achnanthiform 



R. falcatum. 

 R. compactum. 



Rh. falcatum (Donk.) Grun. (Grun. in Hedwigea, 1867; PI. 

 falcatum Donkin ; H. Perag., p. 31, pi. 9, f. 25-27*), plate 28, fig. 801. 



Valves linear, gently sigmoid, with apices obtuse-rounded, decussate, fine ; 

 girdle face gently arcuate, but not achnanthiform. Length, 15 to 18 c.d.m. 

 Breadth, 1 -5 c.d.m. 



Marine. Cresswell, Northumberland ! (Rev. R. Taylor in Coll. W. Arnott, No. 806^). 



Rh. COmpactum (Grev.) Grun. (in Cleve D. from West Ind, 

 Arch., 1878, p. 9; PL compactum Grev. ; H. Per., pi. 10, f. 7-8*), plate 28, 

 fig. 802. 



Valves lanceolate, with sub-acute apices, the upper with raphe straight, not 

 projecting, the lower with raphe projecting very eccentrically on the valve. 

 Transverse striae 20 to 24 in 1 c.d.m., the longitudinal ones being very fine. 

 Girdle view achnanthiform, rather flexed. Length, about 14 to 18 c.d.m, 

 Breadth, about 3-5 c.d.m. 



Marine, Rather widespread (according to Mr. Peragallo), Bahusie (Lagerstedt), 



