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Hexacontium gigantheum Cortese and Bjørklund, 1998

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Hexacontium gigantheum:
PartimHexacontium pachydermum JØRGENSEN 1900, pg. 52 54, no figure.

Description: This species resembles, at a first glance, H. pachydermum in all respects. However, the difference lies in the size of the third cortical shell, which is ranging from 88 to 105pm, with an average of 96.9pm. Furthermore, the number of radial three bladed spines is usually around 10 (always 8 or more). The numerous byspines, 4 6 round each pore, are long and needle shaped, and can be up to 4/5 of the radius of the outer shell, being about 40pm long. Jorgensen (1905) stated that the outer shell could be very thick walled (6 7pm thick), with pores of almost uniform size, around 8pm in diameter. Our description agrees with Jorgensen (1905), and is based on 180 specimens.

Remarks: The diameters of the three lattice shells on one oceanic specimen (Jorgensen 1905) are: 108, 39, and 19pm respectively. These values coincide well with our measurements of this new species, which in our material never has a test size smaller than 88pm. The specimen measured by Jorgensen (1905) cannot be H. pachydermum, which is much smaller.

Discussion: Observing this species for the first time it is quite possible to place it in the genus Actinomma, as the typical 6 spines criterion for the Hexacontium is overridden by the high number of main radial spines. Using the criteria of the microsphere morphology (Dumitrica 1985), it is our opinion that this species cannot be placed in the genus Actinomma. Additional criteria to take into consideration when separating these two genera are the second and third shell shape and pore ornamentation (text fig. 4).

It could be argued that H. gigantheum in reality could be included in H. pachydermum and H. p. forma heptacontia. This is ruled out as the size distribution of these two taxa do not overlap (text figs. 3a b).

The four lateral (L) spines (terminology according to Dumitrica 1985) and the apical spine arise from the primary skeleton, while the antapical (sixth spine) plus all the additional spines arise from the antapical part of the microsphere, according to a not symmetrical pattern.

Holotype repository: PMO 141.552 (Paleontological Museum, University of Oslo, Norway). England finder position of holotype: W49/0, surface sediment from Station 8, Lustrafjord (Bjorklund' s slide collection). Measurements of holotype: 19.3 (first shell diameter), 36.2 (second shell), 97.1 (third shell) µm.
Cortese and Bjørklund 1998


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