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Hexacontium melpomene (Haeckel, 1887)

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Radial portion of the two shells=3:1. Cortical shell thin walled, spiny, with regular, circular pores, four times as broad as the bars; eight to ten on the half equator. Polar spines three-sided, prismatic, ponted, as broad as one pore, only one-third as long as the axis of the sphere (the two shells connected by four thin beams, two opposite in the main axis, two in the equatorial axis).

Dimensions.--Diameter of the outer shell 0.12, pores 0.012, bars 0.003; inner shell 0.04; length of the polar spines 0.04, thickness 0.013.
(Haeckel) 1887
Benson, 1966, p. 141-145; pl. 3, figs. 8-11:

Sylacontarium bispiculum Popofsky

Sylacontarium bispiculum Popofsky, 1912, Deutsche SŘdpolar-Exped., vol. 13, P. 91, Pl. 2, fig. 2.
?Stylosphaera melpomene Haeckel, 1887, Challenger Rept., Zool., vol.. 18, pp. 135-136, Pl. 16. fig. 1

Test with three concentric shells--the cortical shell which is compressed in one axis and has a subquadrate outline, and two medullary shells--all joined by six mutually perpendicular radial beams; two of the beams which lie in a single axis perpendicular to the short axis of the cortical shell are prolonged into two polar spines, oppositely placed, generally three-bladed and unequal to subequal in their length which is about equal to or less than the radius of the cortical shell. The four remaining beams lying in the equatorial plane either terminate at the cortical shell in many specimens or in others continue outward as three-bladed to thin conical spines or thorns which are smaller than the polar spines; in several specimens the junctions of the four equatorial beams with the cortical shell are marked by slight depressions of the latter. Surface of cortical shell with scattered thorns or thin conical spines (up to 18 Ám in length) which arise from the nodes of the intervening bars as well as along the bars. Pores of the cortical shell polygonal to subcircular, subequal to equal in size, about 3-6 times as broad as the bars, without polygonal frames, with subregular to regular hexagonal arrangement, 8-12 on half the circumference. Second shell (outer medullary shell) spherical to subspherical, surface with scattered thorns or thin spines which in several specimens join with the cortical shell to become thin secondary beams with polygonal to subcircular pores, subequal in size, subregularly arranged, 6-9 on half the circumference. Inner (first) shell subspherical to suboctahedral with two to three large polygonal pores on half the circumference; each of the six radial beams arise from the corners of the shell, pierce the second shell, and become heavier and three-bladed. In two tests seven radial beams were observed; in one test one of the beams is not perpendicular to the others; in a few tests one of the polar spines is not collinear with its beam; in one specimen the polar spines are reduced, equal in length to the equatorial spines (7-8 Ám).

Measurements; based on 30 specimens from stations 71 and 81: polar diameter of cortical shell 87-122 Ám (mean, 105 Ám), equatorial diameter of cortical shell 86-114 Ám (mean, 102 Ám); diameter of second shell 36-47 Ám, of inner shell 15-20 Ám; length of longer polar spine 6-55 Ám (mean, 38 Ám) of shorter polar spine 6-43 Ám (mean, 27 Ám), of equatorial spines 0-25 Ám (mean, 11 Ám).

Remarks. This species agrees reasonably well with Stylacontarium bispiculum Popofsky (1912, p. 91, Pl. 2, fig. 2). Popofsky's species has a cortical shell with a subquadrate outline that is compressed in one dimensive axis perpendicular to the axis of the polar spines. It is characterized by only two radial spines oppositely placed in one axis and also by slight depressions where the radial beams of the equatorial plane join the cortical shell, although not all specimens of this species from the Gulf have the latter. It agrees well in size and general proportions but differs from the Gulf species in the presence of conical polar spines and a relatively smooth cortical shell, both of which are subject to intraspecific variation. Most of the specimens from the Gulf have three-bladed polar spines and a thorny or spiny cortical shell. A few were observed with more than two radial spines of approximately the same length, but these were recognizable as S. bispiculum by their compressed cortical shells with subquadrate outline. Most of the Gulf specimens have short thorns or very thin spines which are continuous inward with the four equatorial radial beams, but a few were observed without them, thus conforming to Popofsky's description and illustration.
Haeckel illustrated two species which are similar to S. bispiculum in the presence of two polar spines and a compressed cortical shell of subquadrate outline. Stylosphaera melpomene Haeckel (1887, pp. 135-136, Pl. 16, fig. 1) is identical in outward appearance with the Gulf species except that it lacks the innermost (first) shell and has only four mutually perpendicular beams lying in one plane. Hexalonche octocolpa Haeckel (1887, p. 183, Pl. 22, figs. 6, 6a) also lacks the innermost shell and has six mutually perpendicular radial spines of equal length. The diameter of its cortical shell is also much larger than that of the Gulf species.
The genus Stylacontarium was erected by Popofsky (1912, p. 91) to include three-shelled cubosphaerids with only the polar beams prolonged into spines. Although the presence of just the polar spines is relatively constant in S. bispiculum from the Gulf, this feature is subject to some intraspecific variation as discussed above. Popofsky did not discuss the importance of the compressed nature of the cortical shell and its subquadrate outline, two characters which are constant for S. bispiculum. Since at present Stylacontarium is a monotypic genus, the compressed subquadrangular cortical shell may be of only specific importance. If this genus were placed in synonym with Hexacontium, it would be necessary to emend the definition of the latter which would then invalidate other genera which are based only on characters of the spines. Without reference to the type material of these genera such a revision at this stage is not tenable.

Distribution. This species is of general but rare occurrence throughout the Gulf. It is absent only at the marginal station 99 and at stations 203 and 214 in the northernmost part of the Gulf. It has a slightly but not significantly greater frequency at stations 92, 184, and 192. It appears to be cosmopolitan in the Gulf waters and, therefore, must be fairly tolerant of higher salinites. Its frequency does not increase in regions of upwelling.
Popofsky (1912, p. 91) reported this species from near Madagascar in the western tropical part of the Indian Ocean. Stylosphaera melpomene Haeckel (1887, p. 136) was likewise reported from the Indian Ocean in its eastern tropical part near the Cocos Islands. It thus appears to be a widespread tropical species.

Benson 1966


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