PLEUROSIGMA. 259 



var. arcuatum. {PL areuatum Donk.). 



Differs from the type form by its apices being more suddenly narrowed 

 and moreover flexed, and by its slightly finer strife. 



Marine. England, Chibburn Mouth, Druridge Bay, Cresswell (Donkin), Firth of Tay, 

 Scotland. 



var. prolongatum. {PI. prolongation IV. Sin. .'). 



Much larger and more elongate than the preceding, apices longly 

 produced, often flexed; transverse striae 21 to 23, longitudinal striae 

 finer. Length, 21 to 27 c.d.m. Breadth, 1 to 1*5 c.d.m. 



Marine. Blankenberghe, Antwerp. South Coast of England (W. Sin.). 



This form passes to the following form by every possible transition : 



PI. macrum W. Sm. ! (S.B.D., i., p. 67, pi. 31, f. 276* ; h.v.h. 



Atl., pi. 21, f. 9*), plate 7, fig. 282, and plate 28, fig. 800. 



Valve very narrow, longly lanceolate, with apices feebly sigmoid, rather 

 longly acuminate-rostrate, rostrum narrow. Raphe dividing the valve into 

 equal moieties. Striae delicate, seen with difficulty, the longitudinal ones being 

 25 to 28, the transverse 25 to 27 in 1 c.d.m. Length, 21 to 27 c.d.m. 



Marine. Rare: Antwerp (Scheldt), Blankenberghe. Sussex (W. Sm.), Hull (Xorman), 

 Cheshire (Comber), Ireland (O'Meara), and probably on all the coasts of the North Sea. 



** Frustules enclosed in gelatinous tubes, valves short, thick set. 



PI. eximium (Thwaites), H. Van Heurck {Colhtonema 



eximium Thwaites. Ann. & Mag., 1848, i., pi. 12F. ; pi. 12F; H.V.H. Atl., 

 pi. ai, f. 2* ; Type No. 193), plate 7, fig. 283. 



Valve short, linear, with apices sigmoid, very obtuse. Raphe appearing 



strongly sigmoid, dividing the valve into equal moieties. Transverse stria? 



about 23 to 25, longitudinal stria? 27 or 28 in 1 c.d.m. Length, about 55 

 c.d.m. 



Marine. Antwerp (Scheldt). England, Ireland, France. 



PI. scalproides Rab. (Fl. Eur. Aig., p. 241; h.v.h. Atl., pi. 21, 

 f. 1*), plate 7, fig. 284. 



Valve short, linear-lanceolate, slightly sigmoid, with very obtuse apices. 

 Raphe scarcely sigmoid. Longitudinal striae about 29 in 1 c.d.m ; transverse, 

 about 22, rather more robust than the longitudinal striae, the median slightly 

 radiate. Length, 6 to 7 c.d.m. 



Fresh water. Not yet found in Belgium. Probably only a form of the last. 



