SYSTEMATIC ACCOUNT 3 o 9 



Family RHIZOSOLENIACEAE 



Subfamily RHIZOSOLENIOIDEAE 



i. Cells tubular, valves conical, spine eccentric, sometimes marginal ... ... Rhizosolenia 



2. Cells tubular, valves flat, with marginal spur, connective zone annular ... ... Guinardia 



Genus Rhizosolenia Ehrenberg emend. Brightwell 



Brightwell, 1858 



non Rhizosolenia Ehrenberg, 1843 



The name Rhizosolenia was used first by Ehrenberg (1843, p. 402). One species 

 was described, R. americana, p. 422. Brightwell (1858, p. 93) gave a synopsis of the then 

 known species, and said of the figures of the type provided by Ehrenberg in Mikro- 

 geologie (1854), "most of them certainly not belonging to this genus". Brightwell con- 

 tinued to give a description of the genus Rhizosolenia, and described four new species, 

 the first being Rhizosolenia styliformis. Ehrenberg's type, namely Rhizosolenia ameri- 

 cana, was subsequently transferred to the genus Pyxilla by Grunow in Van Heurck's 

 Synopsis, pi. 836 (1880-85). 



The legal position is that the genus Pyxilla should be referred to Rhizosolenia, and a 

 new generic name should be chosen for those forms which are known as Rhizosolenia. 

 As this step would cause further chaos, and serve no useful purpose, I propose to use the 

 name Rhizosolenia in the sense that Brightwell used it, and to consider Rhizosolenia 

 styliformis Brightwell as the type of the genus, until such time as Rhizosolenia shall be 

 legally conserved. 



This genus is truly planktonic, and with one or two exceptions wholly marine. Its 

 members exhibit the fullest development of the "solenoid" or tubular structure, which 

 is carried out by a complete system of intercalary scale-like segments in the connective 

 zone. The division of the genus depends upon the type and position of the scales, whether 

 they are small and squamose or large and annular, whether they are arranged in dorsi- 

 ventral or lateral lines. The valve portion is usually conical, eccentric and terminated by 

 a spine. I refer to the Rhizosoleniaceae as the "mucronate solenoids". The spine may 

 be solid, or usually with a hollow base, which is connected with the plasma of the cell by 

 means of a fine tube opening into the apex of the valve. In some species the mucro is 

 furnished with small wings, which are often continued down to the apex of the valve 

 proper. These wings may be lateral or dorsiventral. The valves of the cell and some- 

 times portions of the connective zone near the valve bear lines or depressions which 

 correspond to the spine of the sister cell. This is caused by the adpression which takes 

 place within the mother cell while the valves are very young and plastic. The connective 

 zone is usually finely punctate, but the entire cell, in most cases, is so weakly siliceous 

 as to be destroyed if treated with mineral acids. The pervalvar axis of the cell is anything 

 from 4 to 100 times as long as the diameter. 



The chromatophores are usually numerous and cocciform, arranged around the cell 



