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' Mentioned in the text for comparison. ' ?, Data uncertain.

' Mentioned in the text for comparison. ' ?, Data uncertain.

Thalassiosira poroseriata (Ramsfjell) Hasle (Plate 12, Table 15) Basionym: Coscittosira poroseriata Ramsfjell.

References: Ramsfjell, 1959, p. 175, Plates lg and 2a; Hasle, 1972b, p. 544; Fryxell 8c Hasle, 1979b, p. 20, Figs. 31-36; Rivera, 1981, p. 117, Figs. 340-352; Johansen 8c Fryxell, 1985, p. 175, Figs. 10, 51, and 52. Girdle view: Pervalvar axis often longer than cell diameter; valve face flat; mantle low and gently rounded. Several connecting threads about as long as pervalvar axis.

Valve view: Valves usually weakly silicified; areolation radial or fasciculated. One to eight central strutted processes on a straight, curved, or zigzag line; a central areola or annulus at the end of the line; labiate process away from margin almost in a line with the central processes. Distinctive feature: Configuration of central processes. Remarks: Thalassiosira confusa Makarova has central strutted processes in two rows, a labiate process closer to valve margin, and marginal processes more densely spaced than those of T. poroseriata.

Thalassiosira trifulta G. Fryxell in Fryxell 8c Hasle (Plate 12, Table 15) Reference: Fryxell 8c Hasle, 1979b, p. 16, Figs. 1-24. Girdle view: Pervalvar axis shorter than cell diameter; valve face flat; mantle evenly rounded. No observations on chain formation. Valve view: Valves often coarsely silicified; areolae in straight or slightly curved tangential rows. One to eight strutted processes in valve center in one or two lines; labiate process mostly closer to valve mantle than to valve center(?); eight or nine areolae from central process on large valves. Distinctive features: Areola pattern; arrangement of central processes. Remarks: The process pattern is difficult to see with LM because of the heavy silicification of the valves. Thalassiosira frenguellii Kozlova is similar to T. trifulta but it is more weakly silicified; the areolae are smaller, and the marginal strutted processes are more widely spaced. A possible conspecificity of T. trifulta and the Arctic T. latimarginata Makarova was discussed by Fryxell 8c Hasle (1979b) who did not find sufficient evidence to put them into synonomy at that time. This was done by Makarova (1988).

2c. +/— Modified ring of subcentral strutted processes.

Thalassiosira endoseriata Hasle 8c G. Fryxell (Table 15) References: Hasle 8c Fryxell, 1977b, p. 78, Figs. 45-49; Rivera, 1981, p. 68, Figs. 145-157. Girdle view: Valve face flat; mantle low and rounded. No observations on chain formation.

Valve view: Areolation usually fasciculated. Four to 14 central strutted processes in an irregular ring, each process taking the place of an areola inside the middle row of a sector; labiate process about one-fourth the distance from margin toward center.

Distinctive feature: Location of central processes. Remarks: Thalassiosira poro-irregulata Hasle & Heimdal (Hasle &c Heimdal, 1970, p. 573) is another species with an irregular ring of subcentral strutted processes and fasciculated areolation. The internal parts of the processes are much longer and also built differently (SEM) from those of T. endoseriata, and the labiate process is farther away from the margin.

Distribution:

T. confusa—northern cold water region. T. endoseriata—warm water region.

T. frenguellii, T. ritscheri, and T. trifulta—southern cold water region. T. poroseriata—cosmopolitan. T. poro-irregulata—Chilean coastal waters.

Family Melosiraceae Ktltzing 1844

Melosiraceae is characterized by:

Strongly developed pervalvar axes.

A marginal ring of labiate processes (sometimes reduced).

Cells in chains.

Primarily circular valve outline.

Simonsen (1979), who listed these characters, found the family circumscription unsatisfactory and suggested that Melosiraceae should be split into a number of families without doing so himself.

Melosira and Stephanopyxis share the characters mentioned previously; Paralia differs, however, by having a low mantle and a short pervalvar axis. Round et al. (1990) placed Paralia into the family Paraliaceae Crawford in the order Paraliales and Glezer et al. (1988) placed it into the Pseudopodosiraceae (Sheshukova) Glezer. Stephanopyxis, considered synonymous with Pyxidicula Ehrenberg, was placed into Pyxidiculaceae Nikolaev by Glezer et al., whereas Round et al. retained the name Stephanopyxis and placed it in the Stephanopyx-idaceae Nikolaev.

Terminology specific to Melosiraceae

Carina—Collar—a circular membranous costa on the outer side of valve.

Corona—a ring of larger irregular spines at valve apex.

Pseudoloculus—a chamber formed on the outer side of the valve surface by expansion of the distal parts of anastomosing costae.

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