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Actinomma leptoderma (Jørgensen, 1900)

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Echinomma leptodermum:
The outer ball thin-walled (the walls broader than they are thick). The pores polygonally roundish oval, very uneven in size, 7-25 microns, with intermediate walls (2-4 microns broad), which are much broader towards the corners (lumen rounded off).

The middle shell moderately thick (the intermediate walls being as thick as they are wide, about 1 1/2 microns), rather angular and irregular, a little larger than in Hexacontium enthacanthum; diameter about 40 microns. The pores are somewhat uneven, roundish, 4-7 microns. The intermediate walls solid, not particularly broader in the corners.

It is difficult to see the inmost shell, which possesses solid beams (about equal in thickness to those of the middle shell), but rather few polygonal, mostly pentagonal or hexagonal pores, about 8 microns. The diameter of the inmost shell about 15 microns (or a little more).

About 15 main spines, about equally broad inside as outside of the outmost shell, not long. They seldom protrude farther than to a length equal to the distance between the two outer shells, often less, and vary in development. Between the two inner shells, the radial spines are very narrow and in fact hardly wider than the beams of the inmost shell.

The byspines on the outside shell are in appearance like the main spines, but not radially lengthened inwards, with a wide base on the outer shell (like the main spines) and very unevenly developed in size, although generally protruding less than the main spines. Variable in number; although, as a rule, not many, far from being developed in all the corners, only here and there.

The number of the main spines is variable often only about 10, though oftenest about 15. They are 3-edged as in Hexacontium pachydermum.

As in Hexacontium pachydermum and H. enthacanthum, there are forms without outer shells, but there is generally a trace of these in transverse processes on the main spines. These may, however, also be entirely absent. Such forms, of which one is illustrated on pl. VIII f. 33c, might equally well be reckoned as belonging to the genus Actinomma (without byspines on the third shell), respectively Haliomma (with only two shells), if their dimensions and other characteristics were not completely corresponding to the above species. Cfr. Jörgensen 1. c. p. 58.

This species also varies a good deal. When the outside shell is thin-walled, the pores and intermediate walls are of a more uneven size. The byspines are in such cases slightly developed or (as yet) wanting. It is likely that these divergences may be accounted for by a difference in age. A more important difference is the number of main spines, which seems to be able to vary from 10 to 16. Comparatively frequent, though, like all radiolarian with us, always present in small numbers. It occurs, however, decidedly more frequent and in larger numbers than the two Hexacontium species.

Distribution: The same as that of Hexacontium enthacanthum and H. pachydermum. Frequent also on the west coast of Norway and in the Norwegian Sea.
Jørgensen 1905
Description - This subspecies, characterized by its three shells, is one of the most common radiolarian skeletons in fjord sediments, and is also quite common in the plankton. The number of main spines is variable, often only about 10, though most often about 15. They are three-bladed and as long as half the radius of the outer third shell. There are in addition a number of secondary three-bladed spines, starting out from the third shell, having no connection with the second shell. Different forms of E. leptoderma may also have rather strong outer shells and more numerous and longer spines (Jørgensen, 1905).

The outer, third, cortical shell - The outer third shell is relatively thin when compared with the third shell of A. boreale. The pores are round to roundish, of quite variable size, 6-18 µm. The radial spines emerge from the second medullary shell and have about the same length and thickness both inside as outside the third cortical shell. They extend beyond the third shell to approximately the same distance as that between the second and third shell.

The second, medullary shell - The second shell is rather coarse, irregular in outline, a fairly distinctive feature for this species. The pores are roundish and considerably smaller than on the third shell, having a diameter of 4-8 µm. Not all the radial spines that connect the outer third shell and the second medullary shell seem to be connected to the innermost medullary shell.

The first, medullary shell - The first medullary shell is spherical and built up of thin bars forming large polygonal pores.

Remarks - This species varies greatly in both shape and size. When the outside shell is thin-walled, the pores and intermediate walls are of a more uneven size. The byspines are then slightly developed or not present at all.

Distribution - This is a temperate oceanic form (Jørgensen, 1905). It is also reported from the Kara Sea (Bernstein, 1934), Chukchi Sea, the Arctic Ocean (Hülsemann, 1963; Tibbs, 1967), the Greenland Sea (Petrushevskaya & Bjørklund, 1964; Petrushevskaya, 1969), the Norwegian and the Iceland Seas (This paper, Bjørklund, unpubl. data), and on the eastern flank of the Mid Atlantic Ridge west of Ireland (Bjørklund, 1976b).
Cortese and Bjørklund 1998


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