THE BRITISH DIATOMACE^. 



Genus 1. EPITHEMIA, Kutz. 



Frustules adherent^ quadrilateral ; valves arcuate, furnished with trans- 

 verse canaliculi. 



A section of the frustule in the present genus would present an 

 outline of varied form, either elhptical or quadrilateral, or that of two 

 nearly perfect circles united by an oblong isthmus ; the shape of the 

 frustule in the latter case may be compared, as it has been done by 

 Meneghini, to that of a coffee berry, or more correctly to a grain of 

 wheat, but in no case would the section present a trapezoidal outline, 

 the valves being always parallel or equidistant. This genus, as the 

 name implies, is characterized by the adherence of its frustules to 

 Algae of a larger growth. This character is true of most of the species, 

 and even where not strictly applicable, as in E. gibba and others, we 

 detect a disposition to rely on a foreign body for support, their 

 frustules being usually imbedded in the mucus of some member of 

 the family of the PalmellaccEe. 



The transverse costae, so conspicuous in some species, I regard 

 as minute canals, which convey the nutrimental fluid to the surface 

 of the internal membrane. They are not modifications of the cellular 

 structure of the siliceous epiderm, otherwise, the striae would not 

 pass, as they do, uninterruptedly over the entire surface of the valve. 



Conjugation, and the formation of Sporangia in this genus, have 

 been noticed by Mr. Thwaites in E. turgida and E. gibba, by myself 

 in these species and in E. Sorex. 



