ill!Ill

254
PLEUROSIGMA.
PI. acutum Norm. (Prit. Inf., p. 920 ; Per., pi. 3, f. 1, 4*), plate
35. fi g s - 914,
9i5-
Valve elongate, rather broadly lanceolate, very sigmoid, with very acute
apices. Raphe much flexed with eccentric apices. Striae 20-21 in 1 c.d.m.
Length,
16 to 18 c.d.m.
Marine. England (Norman).
PI. marinum Donk. /T.M.S., 1858, vi., p. 22, pi. 3, f. 3 ; Perag.,
pl. 3, f. 11*), plate 28, fig. 794.
Valve narrow-lanceolate, not sigmoid, with apises slightly tapering rostrate.
Raphe having a double flexure in contrary
directions between the central
nodule and the apices. Oblique striae 20 to 21-5, transverse 21*5 to 22 in 1
c.d.m. Length, 12 to 15 c.d.m.
Marine. England (Donkin, Kitton, Comber, Stolt). Ireland (O'Meara). Cherbourg, France
(Brebisson).
AA. Transverse striae delicate, oblique striae robust ; raphe strongly sigmoid.
PI. formosum W. Sm. !
Atl, pl. 19, f. 4* ; Type No. 177), plate 6, fig. 268.
(S.B.D., i., p. 63, pl. 20, f. 195; H.V.H.
Valve linear-lanceolate, strongly sigmoid, with obtuse apices. Raphe
strongly sigmoid, dividing the apices of the valve into two very unequal
portions. Transverse striae rather delicate, 14 to 17 in 1 c.d.m. Oblique
striae very robust, cutting one another at right angles, 10 to 12 in 1 c.d.m.
Length very variable, but usually from 35 to 45 c.d.m.
Marine. Found in fragments in the washings of mussels, Belgium. England, Ireland, France,
and piobably on all our coasts.
Pl. obscurum W. Sm. (S.B.D., i., p. 65, pl. 20, f. 206*), plate
28, fig. 795.
Is distinguished from the preceding, of which it appears to be a miniature
form, by its fine striae, 21 to 25 (according to W. Sm.), and by its very small
size, which only reaches from 8*5 to 15 c.d.m.
Marine. England.
Pl. decorum W. Sm. !
Atl., pl. 19, f. 1*), plate 6, fig. 269.
(Loc. cit., p. 63, pl. 20. f. 196; h.v.h.
Valve very narrowly lanceolate, strongly sigmoid, with acute apices. Raphe
very strongly sigmoid, dividing the apices of the valves into two very unequal