252 PLEUROSIGMA. 



of points sometimes crossing the central nodule, which is large, rounded. 

 Terminal nodules conical, meeting the margins; striae oblique, 17 to 21 in 

 1 c.d.m. ; the transverse striae more distant, 16-19 in 1 c.d.m. 



Marine and sometimes brackish water. On all our coasts. 



PI. affine Grun. ! (H.V.H.,pl. iS, f. 9* ; Type No. 175), plate 6, 

 fig. 263. 



Valve lanceolate, without outline infinitesimally flexed, scarcely sigmoid, 

 with apices sub-obtuse. Raphe feebly sigmoid. Striae iS to 20 in 1 c.d.m., 

 the median striae crossing one another at right angles in the form of slightly 

 flexuous lines, the terminal crossing at an acute angle. Length, 10 to 22 

 c.d.m. 



Marine. Rather rare. Washings of mussels (Deby-), Blankenberghe. England (Kitton). 

 Denmark. 



var. Nicobarica. (PL Nicobaricum Grun., Novara ; H.V.H. Atl. 

 Supp., f. 34), plate 6, fig. 264. 



Distinguished from the preceding by a completely straight raphe, and 

 its sides not sigmoid. 



Marine. Ostend (according to Kitton). Blankenberghe (observed very frequently in a 

 gathering from the mud of the second basin made in April, 1884). 



Mr. Kitton believes that PL affine and its var. JSicobarica are forms of 



Pi. rigidum IV. Sm. ; I, however, think that the flexuous direction so 



characteristic of the median striae allows me to make a special type of 



PL affine. 



var. Normanni. (PI. Normanni Raljs). 



Distinguished from the type form by its more lanceolate shape (more 

 acute) and its rather finer striae. 



England (Kitton, Norman). Ireland (O'Meara). 



PI. navicillaceum Breb. (Diat. Cherb., p. 17, f. 7; PL trans- 

 versalc W. Sm. ; H.V.H. Atl. Supp., f. 35* ; in Type No. 320), plate 6, fig. 

 266. 



Valve lanceolate, with symmetrical margins, apices sometimes very slightly 

 turned in contrary directions. Raphe very flexed with strongly eccentric 

 apices. Central nodule very dilated transversely. Transverse striae 18 or 19 

 in 1 c.d.m. ; median oblique striae slightly flexuous, 13 or 14 in r c.d.m. ; 

 terminal oblique striae 16 or 17 in i c.d.m. Length, 8 to 12 c.d.m. 



Marine. Blankenberghe. Rare. England, France, Holland, Bahusie, Denmark. 



The PL lanceoldtum Donk. approximates closely to the shape of PL 

 naviculaceum, but differs from it in its striation. The transverse striae number 

 20-21, and the oblique 19 in the middle of the valve and 20-21 in 1 c.d.m. at 

 the apices. 



