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Plectacantha sp. Benson, 1966

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Benson, 1966, p. 356-357; pl. 23, figs. 21-23:

Plectacantha? sp.

Cephalis broadly conical, generally open at the top but in a few specimens closed. Cephalis constructed of numerous (6-9 or more) three-bladed to cylindrical ribs which originate from the apical-lateral arches and the ventral arch of the collar ring and diverge upward; one of these ribs represents the apical bar. A lattice of relatively large but unequal, polygonal or subrectangular pores separated by thin cylindrical bars is developed between these ribs; the ribs extend beyond the lattice as spines. Collar ring generally with 3 collar pores, the vertical bar generally absent but present in a few tests; cardinal pores are of type B. The dorsal and primary lateral bars extend as short conical spines, the latter nearly horizontal; two to three short conical branches arise from these spines at or near their origins. A short, axial thorn is present; a short, vertical spine originating from the ventral arch is present in most specimens. In a few specimens one or more of the basal spines is joined to the cephalis by an arched bar.

Measurements; based on 10 specimens from stations 81, 106 and 192: height of cephalis (median bar to tip of spines extending from ribs) 62-123 µm, maximum breadth (between tips of spines) 68-125 µm; length of dorsal and primary lateral spines 12-25 µm.

Remarks. The vertical bar is generally absent in most specimens of this species, but it was observed in a few. When absent this species conforms to the genus Plectacantha Jørgensen, but if present it technically does not. The presence or absence of the vertical bar within a single species makes this character a poor one for defining a genus.
Specimens with the top of the cephalis closed by a lattice to some extent resemble members of the genus Peridium. The cephalis, however, is open at the top in most tests, although these forms may be incompletely developed. The basic structure of the cephalis consists of the collar ring and the upward-divergent spines originating from it, not unlike the spines of numerous specimens of Peridium longispinum from the Gulf. The cephalis in all species of Peridium, however, is completely latticed, whereas of the species of Plectacantha illustrated by Jørgensen (1905), many specimens have an incompletely latticed cephalis. For this reason the species in question was placed tentatively in Plectacantha. Because the generic designation of this species is in doubt, a new species name for it is not proposed.

Distribution. This species is rare in the Gulf but occurs with greater frequency in the northern half. It is common at station 133, absent at stations 27, 60, 71, 90, 91, and 99 in the southern Gulf and at 130, 184, 194, 203, and 214 in the northern Gulf, and rare at the other stations. Its greater frequency in the northern Gulf corresponds to stations located within or near the diatomite facies; therefore, it responds to upwelling only in this region.
Benson 1966











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