ill!Ill

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. !
fig. 263.
(H.V.H.,pl. iS, f. 9* ; Type No. 175), plate 6,
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
terminal oblique striae 16 or 17 in i c.d.m. Length,
slightly flexuous, 13 or 14 in r c.d.m. ;
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.