STAURONEIS. 1 59 



/. Raphe straight. 



A. Valves with margins not undulate. 

 a. VALVES WITH APICES NOT PERCEPTIBLY ATTENUATE ROSTRATE. 



* Stauros very broad, raphe bifurcated. 



S. Phcenicenteron Ehr. (Verb, 1843, pi. 11. 5, f. 1., h.v.h. 



Atl, pi. 4, f. 2*, Type No. 40) ; plate 1, fig. 50. 



Valve lanceolate infinitesimally attenuate up to the apices, which are 

 obtuse, rounded, often slightly constricted ; raphe formed of a double line on 

 the greater part of its length, and surrounded by a broad hyaline zone ; 

 stauros, reaching the margins of the valve, very broad, generally a little dilated 

 n jar its extremities ; striae radiant, about 14 in 1 c.d.m., finer but distinctly 

 monililorm. Length, 10 to 17 c.d.m. 



Fresh water. Rather csmnion everywheie. 



According to Prof. H. L. Smith S. Phcenicenteron is simply a sporangial 

 form of S. gracilis. 



S. gracilis Ehr. (Amer., pi. i, 2, f. 14, W. Sm. ! S.B.U. i., p. 59, 

 pi. 19, 1. 186*; Type No. 41), plate 25, fig. 702. 



Distinguished from the preceding species by its small, narrow valve, its 

 fine striae, 18 20 in 1 c d.m. Length 5.5 to 8.5 c.d.m. 



PVesh water. Belgium, England. 



Observ. Most authors distinguish S. gracilis Elu . from S. gracilis IV. Sm. 

 the latter being characterised by a stauros which does not reach the margins 

 ot the valve. But this character is illusory appearing so in frustules set dry 

 or imperfectly, but in good preparations which we have made with various 

 material from the herbarium of W. Smith, as well as in preparations made by 

 Prof. Smiih himself, this character no longer exists. We have, therefore, 

 referred the form of W. Smith to S. gracilis Ehr., as W. Smith did himself 

 in his Syn. Br. Diat, vol. I., p. 59, and as does also Ralfs in Pritchard, p. 913. 



S. acuta W. Sm. (s.b.d. i., p. 59, pi. 19, f. 187; h.v.h. Atl., 



pi- 4, f- 3*; Type No. 42), plate 1, fig. 51. 



Valves lanceolate, with margins forming a slight cavity from the median 

 portion up to the apices, which are obtuse, and show a very distinct lumen 

 (formed by a thickening of the internal terminal margin) ; raphe formed by a 

 double line throughout the greater portion of its length, surrounded by a 

 very broad hyaline zone ; stauros very broad, dilated near the margins of the 

 valve; striae slightly radiant, about 12 in 1 c.d.m., formed of rather distant 

 puncta. Frustules united in bands of 3 to 6 individuals with girdle view 



