and size, but all agreeing in a few general characters. The most
important of these is the keeled form of the valves, and the remarkable
inequality, in many of the species, between the portions of the
valve lying on either side of this prominency. This inequality, or, in
other words, this eccentricity of the keel distinguishes Nitzschia from
Amphiprora, in ivhich the keel is also present, ivhile the presence of
a keel and its accompanying line or lines of puncta, together with the
absence of any form of stipes, separate the present from the genus
Synedra, with which several of its species have been united by previous
authors. The true character of the valve may be detected by a
careful examination of the desiccated frustule, and is easily seen in
the figures given in Plates X III. and XIV. The fractured valve
shown in PL XIV. fig. 118. a, and the sections given in figs. 115. S.
and 118. S. will more particularly serve to illustrate the form of valve
peculiar to the genus.
Sec t . I. Frustules sigmoid. [Sigmatella, Kiitz.)
1. Nitzschia sigmoidea, W. Sm. F. linear, sigmoid, truncated;
V. linear, suddenly tapering towards the acute extremities ; keel
with a single line of puncta; striæ 85 in -OOl". Length -0116" to
•0192". v.v.
¡3. F. reflexed; striæ distinct, 42 in -OOP'.
Navicula sigmoidea, Ehr. Inf. xiii. 15. Synedra sigmoidea, Kiitz. Bacill.
iv. 37. Nitzschia elongata, Hass. Alg. cii. 12. Sigmatella Nitzschii,
Kiitz. Sp. Alg. p. 18.
Fresh water; very common. Lewes, Dec. 1851, W. Sm.
¡3. Lough Mourne Deposit. Dolgelly Earth.
Plate XIII. 104. 104 ¡3. F. V. of single valve of var. (3. Frustule with
endochrome. Frontispiece, fig. CIV.
2. Nitzschia Brebissonii, W. Sm. F. broader and less sigmoid
than the last on the F. V. ; V. linear ; extremities attenuate, produced,
obtuse ; striæ 27 in -OOl". Length -0034" to -0092''. v.v.
Synedra Armoricana, Kiitz. Bacill. iv. 34. Sigmatella Brehissonii, Kiitz. Sp.
Alg. p. 18. ad specimina quæ misit cl. De Brebisson, Oct. 1852.
Fresh water. Near Lewes, Oct. and Dee. 1852, IF. Sm.
3. Nitzschia Sigma, W. Sm. F. on F. V. linear, gradually tapering
towards the truncated extremities; V. linear on F. V., somewhat
lanceolate, acute ; striæ 56 in -OOl" ; puncta of keel in a double
row. Length -0050" to •0116". v.v.
Synedra Sigma, Kütz. Bacill. xxx. 14.
Marine or brackish water. Pevensey Bay, Sept. 1850, IF. Sm.
Plate XIII. 108.
4. Nitzschia obtusa, n. sp. F. on F. V. linear, with rounded extremities;
V. linear, ohtuse ; puncta double; striæ 56 in -OOl".
Length -0042" to •0121". v.v.
Brackish water. Shagalieu Marsh, Poole Bay, June 1849, IF. Sm.
Plate XIII. 109.
5. Nitzschia spectabilis, n.sp. F. on F.V. linear, sigmoid, truncate
; keel nearly central ; puncta in 4 rows, the more distant on
each side larger ; V. striated, v.s.
Brackish water. Near Ipswich, Mr. Hodgson, Aug. 1851.
Plate XIV. 116.
Sec t. II. Valves linear, straight.
6. Nitzschia scalaris, W. Sm. F. V. linear, inflated towards the
truncated extremities ; V. linear, slightly tapering towards the extremities,
which are somewhat acute; keel eccentric; puncta
double, alternately produced into shorter and longer striæ ; surface
of valve distinctly striated ; striæ 24 in •OOl". Length •0125" to
•0172". v.v.
Synedra scalaris, Kütz. Bacill. xxviii. 32.
Brackish water. Shagalieu Marsh, Poole Bay, June 1849, IF. Sm.
Plate XIAL 115. Ideal section of frustule, fig. 115 s.
7. Nitzschia linearis, W. Sm. F.V. linear, with rounded or truncated
extremities, occasionally with a slight central constriction ;
V. linear, with acuminate extremities ; keel nearly central ; puncta
in a single row ; striæ obscure. Length •0023" to •0075".
Frustulia linearis, Ag. ex sp. authen. quæ misit cl. Dr. Greville.