35 



TABELLARIA. 



B. Entetracyclus (Ra/fs). 



T. lacustris Ralfs (in Ann. and Mag. Nat. Hist., xii., pi. 4, f. 

 H.V.H., in Type No. 349). 



2 ; 



Fig. 108. Tetracyclus lacustris. 



Valve more or less cruciform or elliptic, lanceolate, with median portion 

 strongly ventricose, venter rounded, showing 4 to 1 2 costae, transverse, curved 

 near the median portion ; striae moniliform, excessively faint, and scarcely 

 visible, about 25 to 30 at the middle of the valve, pseudo-raphe narrow. 

 Frustulcs in long filaments, showing numerous septa. 



Fresh water. England, Wales (Ralfs.), Scotland (Gregory), Ireland (Win. Sm.) 



var. marginata (Ehr.), W. Sm. {Tetracyclus eniargina/us, IV. 

 Sm. ; Biblarium emarginatum Ehr.) 



Valve more regularly cruciform, with inflation excavated. 



Fresh water. England. Scotland (Grew, Greg.), Ireland (W. Sm.). 



