298 Badllariece 



the sagittal axis and in some forms consisting of distinct beads. 

 In the girdle-view the frustules are cuneate in outward form. 



There are about 14 British species of the genus, some of which, such as 

 G. constrictum Ehrenb. (fig. 140 C), G. acuminatum Ehrenb., G. parvulum 

 Ktitz. and others, are common in every part of the British Islands, frequently 

 occurring as epiphytes on other Algse. G. geminatum (Lyngb.) Ag. is the 

 largest species (length of valves 90 120 /a; greatest breadth 35 40^; fig. 

 140 A and B) and is principally confined to hilly districts, often forming 

 thick felt-like masses of a greyish-white colour on dripping rocks. 



Genus Rhoicosphenia Grun., 1860. In the valve- view the 

 frustules are similar in form to those of Gomphonema, being 

 fusiform with dissimilar poles ; in the girdle-view they are cuneate 

 and considerably curved. The valves are dissimilar, the upper 

 valve only possessing a pseudo-raphe and no nodules, whereas the 

 lower valve possesses a raphe and central and polar nodules. 



R. curvata (Ktitz.) Grun. is common all over Britain, often clothing in 

 dense masses the thicker branches of species of Cladophora ; length of valves 

 1345 M . 



Family 3. COCCONEMACEJE. 



The family is principally characterized by the curvature of the 

 frustules in the plane of the girdle, so that the valve-view always 

 appears bent or sublunate. The girdle-view is straight and 

 generally subrectangular. The frustules are symmetrical about a 

 longitudinal plane and a median transverse plane. Each valve 

 possesses a raphe, sometimes straight, but more often curved, and 

 situated more or less near the concave margin. There are distinct 

 polar and central nodules except in Epithemia. There is one large 

 parietal chromatophore in each cell, the median part of which is 

 disposed within the concave girdle-face. Two auxospores arise 

 without conjugation from two mother-cells, growing side by side 

 as elongated rejuvenized cells. 



There are three British freshwater genera : 



A. Valves without transverse ribs. 



* Cells not very asymmetrical ; raphe removed from 



the edge of the girdle ; girdle small, without striae Cocconema. 

 ** Cells strongly asymmetrical ; central nodule very near 

 the girdle on the concave side ; girdle broad, with 

 longitudinal striations Amphora. 



B. Valves with transverse ribs ; raphe often strong and conspicuous. 



Epithemia. 



