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Hexacontium arachnoidale Hollande and Enjumet, 1960

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Benson, 1966, p. 156-158; pl. 4, figs. 6-7:

Hexacontium cf. heteracantha (Popofsky)

?Hexalonche heteracantha Popofsky, 1912, Deutsche SŘdpolar-Exped., vol. 13, pp. 88-89, text fig. 3.

Cortical shell spherical to subspherical, with wall of intermediate thickness; surface with single thorns or conical spines at all or most of the nodes of the intervening bars; in one specimen spines distally bifurcated at a common distance from the cortical shell; pores relatively large, polygonal to subpolygonal, unequal, with irregular arrangement, without polygonal frames, 6-9 on the half circumference; around base of each main spine are three smaller, subequal, circular to subcircular pores, each located between the blades of the three-bladed spine. Second shell spherical to subspherical, in a few specimens globular, with surface nearly smooth to one with numerous scattered short thorns or thin conical spines; with subequal, subregularly arranged, subcircular to subpolygonal pores, without polygonal frames, separated by relatively thin intervening bars, 8-10 on the half circumference. First shell subspherical to polyhedral with relatively large subpolygonal to polygonal pores, 3-4 on the half circumference. Six radial beams, in most tests mutually perpendicular, arise from the surface of the first shell as thin cylindrical beams, become heavy and three-bladed after piercing the second shell; before meeting the inner surface of the cortical shell they are trifurcated with each branch in the plane of one of the three blades of the main spines, branches rejoin slightly above the surface of the cortical shell and continue as one three-bladed main spine (each branch resembles a flying buttress above the surface of the cortical shell). Main spines of equal length and breadth, as long as the radius of the cortical shell in a few specimens. In several tests there are one to three beams and spines in addition to the six mutually perpendicular beams and spines.

Measurements; based on 30 specimens from stations 27, 34, 46, 56, 60, 71, and 81: thickness of cortical shell wall 4-9 Ám; diameter of cortical shell 140-189 Ám, of second shell 47-55 Ám, of first shell 18-25 Ám; length of main spines 25-86 Ám.

Remarks. This species differs from Hexacontium cf. heracliti (Haeckel) in the absence of nodal clusters of byspines at the nodes of the intervening bars of the cortical shell, in the presence, instead, of single, thin conical spines at the nodes, in less heavy spines, and in a less thick cortical shell wall. It differs very little from Hexalonche heteracantha Popofsky (1912, pp. 88-89, text fig. 3) except for the lack of the first or innermost shell. Popofsky, however, states that the single specimen he described was broken. If the innermost shell was present in his specimen but subsequently lost, his species is not different from the Gulf species. Since doubt exists as to the presence of the innermost shell, identification of the Gulf species with H. heteracantha is tentative.

Distribution. In the Gulf this species is absent north of station 184 as well as at stations 64, 90, 95, 99, 133, and 151. It is rare at all other stations except at 130 where it is apparently common (2.8%), although the total population is only 179. It appears to be an oceanic species, but its common occurrence at station 130 is anomalous since it is absent at other marginal stations. Although the dilution of radiolarians by terrigenous sediment is high at this station, this species must be able to produce a sufficient number of tests in order for it to be common. Upwelling along the eastern shore of the Gulf probably is important in this respect.
Popofsky (1912, p. 89) reported this species from the tropical Atlantic; therefore, it may be cosmopolitan in the tropical parts of the oceans.

Benson 1966


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