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Volume 86, 1959
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Genus Pleurosigma W. Sm., 1853

Cells solitary, free; frustules in valve view sigmoid. Valves with central and terminal nodules; raphe usually sigmoid, often more so than valve margin; cell surface delicately marked by longitudinal or oblique and transverse striae; some authors maintain two genera, Pleurosigma wtih oblique, Gyrosigma with longitudinal and transverse striae.

Key to the Genus Pleurosigma
1. Frustules with definite angle on margins on transapical axis. P. angulatum.
Margin evenly curved 2.
2. Raphe straight in central portion P. fasciola.
Raphe sigmoid, central portion curved 3.
3. Frustules evenly sigmoid P. formosum.
Frustules sigmoid towards extremities 4.
4. Raphe evenly sigmoid P. balticum.
Raphe more strongly sigmoid at apices P. naviculaceum.

221. Pleurosigma naviculaceum Breb. (Pl. 6, Fig. 78.)

Breb., 1867, 17.

Boyer, 1916, 74, 22, 6; 1927, 471.

Valves lanceolate, slightly sigmoid at the subacute ends. Raphe nearly central, sigmoid near ends. Striae transverse and oblique, the latter closer at the ends. Central nodule dilated transversely. Length, 80μ.

Distribution. Australia: Heron Island, Lake Conjola. New Zealand: Hauraki Gulf, Wellington Harbour.

222. Pleurosigma angulatum (Queckett) W. Sm. (Pl. 6 Fig. 79a, b.)

W. Sm., 1853, 65, 21, 205.

Boyer, 1916, 74, 22, 3; 1927, 471.

Hend., 1951, 61, 11, 8 (synonymy).

Navicula angulata Queckett, 1848, 438, 7, 4–7.

Cells solitary; frustules in valve view sigmoid-rhomboid, with at times a definite angle on valve margin adjacent to central nodule; raphe sigmoid, central; central nodule small, more or less rhombic; valve surface obliquely and transversely striated. Length, 80–120μ. This form is often smaller than specimens of P. angulatum received from Hendey from Great Britain, and Hendey (in litt.) considers the Australasian form is a distinct species. However, because of considerable variation in form we hesitate at this time to separate our form from P. angulatum.

Distribution. Estuarine-nertic; marine, benthic, epontic, and planktonic, found in algal and Zostera washings, and on fouling test plates. Australia: Moreton Bay, Port Stephens, Lake Macquarie, Port Jackson, Botany Bay, Port Hacking, and from coastal plankton stations of New South Wales. In mud at 135 in off Jibbon Cape (living form). New Zealand: Wellington Harbour.

223. Pleurosigma balticum (Ehr.) W. Sm. (Pl. 6, Fig. 80a, b.)

W. Sm., 1853, 66; 22; 207.

Hend., 1951, 61, 11, 9.

Gyrosigma balticum (Ehr.) Cl., 1894, 118.

Boyer, 1916, 75, 23, 2; 1927. 456.

Cells solitary; frustules in valve view sigmoid towards extremities. Raphe sigmoid and slightly curved close to central nodule; striae longitudinal and transverse. Length 80–182μ × up to 27μ wide.

Distribution. Estuarine. neritic, common in plankton on fouling plates, algae and sea grasses and as a benthic form. Australia: Moreton Bay, Lake Macquarie, Port

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Stephens, Port Jackson, Botany Bay, Port Hacking, Eden, Port Philip. New Zealand: Hauraki Gulf. From Australia by Østrup (1910).

224. Pleurosigma fasciola (Ehr.) W. Sm. (Pl. 6, Fig. 81.)

W. Sm., 1853, 67, 21, 211.

Hend., 1951, 62.

Gyrosigma fasciola (Ehr.) Cl., 1894, 116.

Boyer, 1927, 463.

Cells solitary, sigmoid, extremities drawn out into long, curving beaks; raphe central, straight except at the beaks, where it is sigmoid; striae very fine, transverse and oblique. Length, 60μ.

Distribution. Neritic, marine, epiphytic. Australia: Port Hacking; rare.

225. Pleurosigma formosum W. Sm., 1853 (Pl. 6, Fig. 82a, b.)

W. Sm., 1853, 63, 20, 195.

Boyer, 1916, 73, 22, 5; 1927, 467.

Hend., 1951, 62, 11, 6.

Cells solitary; frustule in valve view sigmoid; raphe strongly sigmoid, sweeping from the central nodule towards the valve margin, giving a very twisted appearance; striae transverse and oblique. Length, 100–180μ × up to 20μ.

Distribution. Estuarine-neritic; epontic on weeds and fouling test plates, common, frequently benthic, and occasionally planktonic. Australia: Heron Island, Moreton Bay, Lake Macquarie, Port Jackson, Botany Bay, Port Hacking. New Zealand: Hauraki Gulf, Lyall Bay (Petit 1877).

The following additional species of Pleurosigma have been recorded previously:—

  • P. compactum Grev., 1857 (Gyrosigma) (Østrup, 1910, 31). Australia. (As Donkinia compacta) (Gr. & St., 1887, 133). Oamaru.

  • P. cuspidatum Cl., 1894. (As P. lanceolatum cuspidatum Cl., 1881.) Type loc. Port Jackson.

  • P. elongatum W. Sm., 1853 (Hardy, 1910). Kew, Victoria.

  • P. kuetzingii Grun., 1860 (Gyrosigma) (Østrup, 1910, 31). Australia.

  • P. nubecula W. Sm., 1853 (as P. intermedium Sm., 1853) (Petit, 1877, 28). Lyall Bay.

  • P. marinum Donk., 1858. (Petit, 1877, 28.) Lyall Bay.

  • P. reticulatum (as Donkinia) Norman in Pritch., 1861, 921.

  • P. validum Shadb., 1854. (Petit, 1877, 28.) Lyall Bay, Foveaux Strait. (= P. rigidum.)

  • P. vitraeum Cl., in Cl. & Grun., 1880 is Navicula vitraea Cl., 1894. (Cl. & Grun., 1880, 60.) Australia.

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