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Hexacontium laevigatum Haeckel, 1887

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Cortical shell thick walled, quite smooth. Pores regular circular, with double margins, eight to ten on the radius, of the same breadth as the smooth bars. Radial proportion of the three spheres = 1:2:6. All three spheres connected by six very thin radial bars, which are porlonged outside into six short, stout, triangular pyramidal spines, half as long as the radius of the outer shell.

Dimensions.--Diameter of the outer shell 0.13, middle 0.04, inner 0.02; cortical pores and bars 0.008; length of the spines 0.04, basal breadth 0.02,
Haeckel 1887
Benson, 1966, p. 153-155; Plate 4, figures 4-5:

Hexacontium laevigatum Haeckel

Hexacontium laevigatum Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 193, Pl. 24, fig. 6.

Cortical shell generally spherical to subspherical but ovoid or ellipsoidal or with subquadrate outline in several specimens; surface generally smooth except in several specimens with a concentration of short, thin, conical by-spines around one of the six main spines, the larger spine in those tests with one spine larger than the remaining five; pores circular to subcircular, small, equal, as wide as or slightly less than the width of intervening bars, with generally regular hexagonal arrangement, without polygonal frames, 12-16 on the half circumference. Second shell spherical to subspherical, surface smooth to slightly thorny, with subcircular to subpolygonal, subequal pores with subregular arrangement, 6-8 on the half circumference. First shell subspherical to subpolyhedral, thin-walled, barely visible in many specimens, with 2-3 large polygonal pores on its half circumference. Six, rarely five or seven, mutually perpendicular radial beams that arise from the surface of the inner shell are heavy and three-bladed between the second and third shell and extend beyond the cortical shell as three-bladed, in a few tests conical, spines or thorns. In most specimens the six spines are of nearly equal length, but in several, one spine is longer (up to 30 µm) and heavier, located at one of the poles of the major axis if the cortical shell is ellipsoidal or ovoid, and surrounded by numerous thin conical by-spines, the remaining five main spines being shorter (as short as 8 µm) or present as short thorns. In a few tests one or more of the beams and/or spines are not mutually perpendicular to the others.

Measurements; based on 30 specimens from stations 71, 81, and 92: diameter of cortical shell (including average of larger and smaller diameters of ovoid or ellipsoidal shells) 77-103 µm, of second shell 30-46 µm, of first shell 12-17 µm; length of main spines 10-65 µm.

Remarks. The representatives of this species from the Gulf differ from Haeckel's description of Hexacontium laevigatum Haeckel (1887, p. 193) in slightly smaller diameter of the cortical shell and in the general absence of double-rimmed pores. One specimen from the Gulf, however, was observed with double-rimmed pores; therefore, this feature is subject to intraspecific variation but is not general within the species. The length of the main spines is also highly variable, but specimens were observed with short spines similar to those shown in Haeckel's illustration (op. cit., Pl. 24, fig. 6). Haeckel does not mention the occurrence of numerous thin, conical spines concentrated at one pole of the cortical shell.
This species differs from H. enthacanthum Jørgensen by its generally smooth surface, its smaller diameter, the presence of more numerous and smaller, more nearly equal circular pores of the cortical shell, the presence of numerous thin by-spines concentrated at one pole of the shell, and in the presence of unequal main spines in numerous specimens.

Distribution. This species occurs very rarely at all stations in the Gulf except 90, 95, 130, 203, and 214 where it is absent. As it has no significantly greater frequency at stations 106, and 192, it is therefore an oceanic form that tolerates slightly greater salinities.
Haeckel (1887, p. 193) reported H. laevigatum from the South Atlantic at "Challenger" station 332. Thus it may be a species that is cosmopolitan in tropical or subtropical seas.

Benson 1966











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