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Plectacantha oikiskos Jørgensen, 1905

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Plectacantha oikiskos:

Periplecta intricata (CL.) JÖRG. L. 90, p. 73.
Peridium (?) intricatum CL. L. 30, pl. 2, f. 8 a, b?
Peridium (?) laxum CL. 1. c, pl. 2, f. 9 a, b.

The primary verticil of branches of the apical spine has (generally?) only two branches, pointing upwards and outwards in the angle between the dorsal, basal spine and the lateral ones. These two branches together with the protruding middle stem form three undivided spikes, about equal in length.

The left, lateral spine, Ll, Has the primary verticil complete. Of these three branches, however, the inside one is – as previously mentioned – transformed onto a short, strong arch, pointing downwards, at the end of which the left, lateral arch and the primary, ventral one meet. The two other branches form with the protruding middle stem, three, diverging, long, single spikes, pointing obliquely outwards and to the sides. By these three long, protruding spikes, the left, lateral spine may always be easily recognized.

The right, lateral spine has only retained one branch of the verticil in its original form, the other two being branches for the two corresponding, primary arches, one belonging to the ventral arch, the other to the right, lateral one. There are consequently here two long, simple spikes, about equal in length, the outer branch in the primary verticil and the protruding, middle stem.

The dorsal, basal spine has only one simple branch of the verticil, pointing outwards and upwards, the two others being strong, arched branches, diverging nearly at right angle on both sides.

One of these branches forms the right, the other the left lateral arch. The middle stem of the basal spine is here branched, there being one or two similar secondary arches outside the primary, lateral ones. The dorsal, basal spine, therefore, only shows one simple, protruding spike. A similar, smaller byspine is generally to be found father out.

The dorsal spine is the longest and strongest, the apical one the shortest and weakest.

Outside the primary meshes, secondary ones are developed more or less richly, varying considerably in size. These unite to form a “network” which is generally rather imperfect, but would seem, on older individuals, to be made more complete by the addition of connecting beams across the larger, secondary meshes, so that these meshes finally are smaller in size and more in number. Generally speaking, the network is conically expanded downwards, and seems, in the case of older individuals, finally to be imperfectly closed beneath by the development of a few, fine, long, transverse beams. In the material examined specimens with such nearly closed network were always rare.

Of the more important byspines and secondary meshes, I will only mention the following which seem always to be present, and may serve to help in identifying the species. Under the protruding right lateral spine a large, secondary, pentagonal mesh will be found. The left, lateral arch has strong byspines in the middle, pointing outwards and upwards, and under it, there is a large, secondary mesh. On both sides of the dorsal spine, outside the primary, lateral arches, on the left side will be found two secondary meshes, and on the right side one; the latter being the larges, but it is prehaps later on divided into two.

Moreover, on all the stronger arches, there is a more or less rich development of byspines, pointing outwards and upwards, sometimes twins, directed upwards-outwards and downwards-outwards.

The beams are thin in young individuals, especially at the outer secondary meshes; they are much stronger on older individuals. The length of the basal, dorsal spine is +/- 55µ.

Cfr. Jörgensen 1900.

I considered this species first to belong to the genus Periplecta HCK.; but as I later on found that what with HAECKEL characterizes this genus is also characteristic of all closely related genera, as I am acquainted with them from plankton samples (not drawings), I have been obliged to change the name of the genus.

I at first considered this species to be CLEVE’s Peridium intricatum, and this may be correct, but it cannot be proved to be so from CLEVE’s illustration nor from his description. As it is, moreover, quite as probable that CLEVE’s Peridium (?) laxum also belongs to this species, I consider it best to retain the manuscript name I originally had given the species before CLEVE’s work was published.

Not rare, but always in small numbers.

Distribution: Not rare on the west coast of Norway, always scarce. Probably boreal oceanic.
Jørgensen 1905


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