STELLADISCUS. 505 



GENUS 168. STELLADISCUS RATTRAY, 1890. 



Valve circular, without rosette 

 or central space. Rays straight, 

 clavate, arising from the hyaline 

 centre, where they are much 

 elongated. Marginal portion 

 (occupying the outer two-thirds 

 of the valve) divided into 12 

 punctate compartments. 



This genus only comprises a 

 single species, Stelladiscus Stella 

 Ratt. (Asterolampra Stella, Norm.), 

 figured in the text after Norman. 



Fig. 252. Stelladiscus Stella 



This diatom should be found at Sierra Leone, according to Norman, who, 

 however, only saw one specimen, and no one since him has again seen this 

 singular form. 



Asterodiscus, Johns, 1852, in Amer. Journ. So., 1852, p. 33, is a very 

 doubtful genus. According to the author it should be characterised by 

 similar valves, with central ray bifurcated, and there should be three species, 

 A. quinarius, senarius, and nonarius, according as it has 5, 6, or 9 rays. As 

 these forms are not figured it is impossible to know what Johnson has seen. 



GENUS 169. RYLANDSIA GREV., 1861. 



Valve disciform, with cellular 

 structure, cells interrupted by two 

 opposite rays surrounded by a hyaline 

 space, very dilated at the base, not 

 reaching to the centre of the valve. 



A single species : R. biradiata Gr, 

 (fig. 253), which is found in the Barbadoes 

 deposit. It is a very rare form ; the figure 

 in the text was drawn from one of my 

 photographs. 



Fig. 253. Rylandsia biradiata. 



