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Larcospira minor (Jørgensen, 1900)

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Lithelius minor
Spirale doppelt; jede nur mit 1 1/2 Umgängen. Länge 0,065 mm., Breite 0,058.
Markschale kaum 0,011 mm. breit. Aeussere Poren etwas ungleich gross, rundlich bis oval, dickwandig. Ungefähr 150 lange, schmale Radialstacheln, die längsten ungefähr 0,030 mm. ( = d. Radius) oder mehr hervorragend.

Diese kleine Art steht dem Lithelius solaris HCK. in allen Hinsichten sehr nahe, unterscheidet sich aber ausser durch die viel kleineren Dimensionen auch durch nur 1 1/2 (statt 4) Umgänge der Spiralen. Auch sind die Spiralgänge fast gleichbreit, nur wenig breiter nach aussen.

Nicht selten, immer spärlich vorkommend: Mai—September, November.
Jørgensen 1900
Larcospira minor:
This species has a very difference appearance, according to the position in which it is seen. In one position it has an appearance corresponding to the illustration referred to, and which caused me to consider it as a Lithelius with a double spiral. On being rolled under the microscope it has, in another position, the appearance of 3 distinct (concentric) shells, one outside the other, the innermost being somewhat oblong, the other two rounder. In this last position, an indefinite contour of an inner, smaller shell is seen in the innermost one. From which one may probably conclude that the innermost of the three shells is double, Larnacilla-shaped or trizonal (according to Haeckel's designations). I have not, however, succeeded in seeing this clearly. From this shell there extends, on both sides, a transverse girdle. Which winds itself into a spiral about the largest axis of the inner shell, the longitudinal or principal axis (after Haeckel). These two spirals gave rise to the appearance of a Lithelius with a double, when looked at from above (transverse section, after Haeckel apical view). In a certain, a little oblique, position one gets again a more or less indefinite impression of a single spiral. In each spiral there is only a little more than one turn. Pores somewhat uneven, not large. Numerous, long, narrow, needleshaped radial spines. One of the most frequent radiolaria in the north, sometimes also rather numerous, especially in ocean samples.

Distribution: Also rather frequent on the west coast. Known too from a few places in the in the Norwegian Ocean, where it occurs together with southern forms.

NOTE: What is in one instance in the tables entered as Lithelius spiralis Haeckel is very uncertain, and it is probably only a form of Larcospira minor, which, as already mentioned, in certain positions gives the appearance of a single spiral. The same is the case with regard to L. spiralis Jørgensen (1900) p. 66.
Jørgensen 1905
Benson, 1966, p. 262-265; pl. 17, figs. 9-10; pl.18, figs. 1-4:

Lithelius minor Jørgensen

Lithelius minor Jørgensen, 1900, Bergens Mus. Aarbog (1899), pp. 65-66, Pl. 5, fig. 24.
Larcospira minor Jørgensen, 1905, Bergens Mus. Skrifter, p. 121.

Ellipsoidal to spherical test consisting of 3-7 or more concentric trizonal shells, separated by approximately equal distances (6-25 µm); in certain orientations internal structure appears as a double spiral. Shells supported by thin radial beams which arise from the nodes of the intervening bars of the lattice, therefore, several hundred in number; outermost shell with thorns or thin conical spines (incipient beams for support of an additional shell) arising from the nodes of the intervening bars; spines generally short (5-20 µm), but long (up to 40 µm) in a few specimens. Pores of all shells of nearly the same size, with regular to subregular arrangement, subcircular to subpolygonal, 9-15 on half the minor circumference of the outermost shell.

Measurements; based on 30 specimens from station 71: major diameter of test 79-148 µm minor diameter 70-132 µm.

Remarks. Jørgensen (1900, pp. 65-66) first placed this species within the genus Lithelius Haeckel because of its double spiral appearance in a certain orientation. Later observations by him (1905, p. 121) revealed that when specimens of this species are rolled under the microscope the internal structure appears as three distinct concentric shells in certain positions, in others as a double spiral, and in a certain oblique position as a single spiral. On the basis of these observations he placed the species within the genus Larcospira Haeckel because of the presence of a double, probably trizonal, inner shell. Because all the concentric shells of this species are trizonal, the innermost ones are also; therefore, the presence of a double trizonal inner shell loses significance. Because Haeckel's only illustrated species of Larcospira is very different from this species, Jørgensen should have retained the generic name Lithelius because species showing double or single spirals, whether truly coiled or consisting of concentric trizonal shells, were placed by earlier workers in this genus.

Lithelius spiralis Haeckel (1862, pp. 519-520, Pl. 27, figs. 6, 7) and Lithelius alveolina Haeckel (1862, p. 520, Pl. 27, figs. 8, 9) are similar to the Gulf species but differ from it in having a single, not double spiral. Without reference to type material of this species, identification is not positive. Some specimens from the Gulf in certain orientations appear to have a single spiral, but on closer inspection the double spiral becomes evident.

Distribution. This species is cosmopolitan in the Gulf, being absent only at stations 192, 203, and 214. Its highest frequency (5.8%) was observed at station 27 where it is common and is the third most abundant species. It is common at stations 56, 60, 71, 81, 90, 92, 93, 95, 99, 115, 133, and 151. It has a greater frequency in the southern half of the Gulf, especially in the axial and western portions, although it is common at station 99 which is located in an area of upwelling. Except at this station, the distribution of this species is apparently controlled by upwelling along the Baja California coast. In the northern half of the Gulf it is common at three stations located within the diatomite facies, namely 115, 133, and 136, further evidence of its response to upwelling. Its generally common occurrence in the southern Gulf and its gradual decrease northward indicates its preference for more nearly oceanic water messes.

Jørgensen (1905, p. 121) states that this species is one of the most frequent radiolarians in the North Atlantic, being sometimes numerous in ocean samples. It is also frequent off the west coast of Norway and is known from a few places in the Norwegian Ocean where it occurs with southern forms. This species, therefore, appears to be cosmopolitan in its distribution, occurring at all latitudes.
Benson 1966


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