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Trigonopyxis microstoma
Trigonopyxis microstoma, after Hoogenraad and De Groot, 1948

Trigonopyxis microstoma Hoogenraad and De Groot, 1948

Diagnosis: Shell relatively large, circular in dorsal view, sometimes elliptical. The ventral side of the shell is flat and only in the center there is a small depression with the aperture which is small and irregularly-shaped, surrounded by a more or less organic lip.

Dimensions: Shell diameter 90-127 μm, height 52-60 μm, diameter of the aperture 10-17 μm, rim 1-4 µm thick.

Ecology: Sphagnum, moist mosses. Patagonia.

Trigonopyxis microstoma
Trigonopyxis arcula
T. microstoma, mounted in balsam by De Groot, 1953, and labeled Tierra del Fuego (private collection Ferry Siemensma)
Trigonopyxis microstoma
T. microstoma, after Hoogenraad and De Groot, A=1951, B=1942

Original description:

Trigonopyxis microstoma nov. spec.

In a rather considerable number of individuals the sample contained an object we believe to be the test of a hitherto unknown species of Rhizopods, afterwards we discovered the same object again in moss samples from Tierra del Fuego in a nearly identical form. Shape and structure of the object in general as well as the general features of the aperture reminded us so strongly of those of Trigonopyxis arcula, that under reserve of possible later corrections we consider this form to be an as yet unknown species of the above named genus Trigonopyxis Penard. Some delicate characteristics of structure, but especially the different form and much smaller dimensions of the aperture are sufficient to distinguish it from the hitherto only known species of the genus: T. arcula (Leidy) Pen. (= Difflugia arcula Leidy). On the ground of the most salient feature of the test., viz. the small aperture, we provisionally propose for the species the name: Trigonopyxis microstoma.
Seen from above, i.e. in the direction of the oral-aboral axis, the test is nearly always exactly circular in outline, sometimes, however, slightly elliptical; seen from aside the test appeared as a rather strongly compressed spheroid, with a height of almost half the diameter of the test. The oral plane was usually less flattened than the fundal one; the aperture more or less deeply, sometimes, however, hardly, invaginated. The colour varied from yellowish to rather dark brown. The fundal part was covered with xenosomes of all sorts, mainly larger and smaller quartz grains and variegated sapropelium particles, sometimes projecting far beyond the outline of the test; among them often projecting almost completely beyond the circumference some circular, colourless corpuscles of 10—15 µm diameter, such as are also often observed in the test-covering of other Rhizopods, especially in species of the genus Amphitrema; probably they represent the empty cell walls of certain unicellular Algae. Never were Diatomaceae tests observed among the test elements; they also failed in the sample itself. By way of exception, the sapropelium particles also failed entirely and the covering existed only in a dense collection of quartz elements; in this way the test obtained the appearance of those of some Difflugia species. The xenosome-covering of the oral side was always much less dense than that of the fundal one; sometimes it lacked almost completely and in that case the ground structure of the test was clearly perceptible in that place as a fine, rather uniform granulation.
By treating the test with a dilute KOH-solution, a smaller or greater part of the xenosomes came off; tests treatened in this way allowed a close study of the size and form of the aperture and the structure of its margin and surroundings.
This aperture varies only little in size, but very much in form: irregular round, or, owing to the formation of obtuse angles, polygonal; never was the typical triangular form of the aperture observed which. T. arcula shows usually and to which the genus owes a part of its name.
The margin of the aperture showed itself in some cases only as a scarcely thickened, dark line; usually, however, as in T. arcula, a distinctly developed, thickened, dark brown coloured rim was present. Sometimes it seemed as if this rim bored on its exterior margin a number of irregular placed, very fine grains, which gave them a more or less wart-like appearance.

Remains of the protoplasmatic content were never observed.
Dimensions: diameter of the test (as seen in the direction of the oral-aboral axis) 90—127, height 52—60 µm; aperture 10—17 µm, breadth of the rim 1—4 µm.

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