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Circodiscus microporus (Stöhr, 1880)

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Benson, 1966, p. 207-210; pl. 9, figs. 7-8; pl. 10, fig. 1; text-fig. 12:

Ommatodiscus pantanellii Carnevale

Ommatodiscus pantanellii Carnevale, 1908, Mem. R. Ist. Veneto, vol. 28, no. 3, pp. 24-25, Pl. 4, fig. 6.
Ommatodiscus simonellii. Carnevale, 1908, op. cit., p. 25, P1. 4, fig. 7.
Ommatodiscus reniformis Carnevale, 1908, op. cit., p. 25, P1. 4., fig. 8.
Ommatodiscus circularis Carnevale, 1908, op. cit., p. 26, Fl. 4, fig. 9.

Test a flat disc, circular or subcircular to elliptical in outline in fully-developed forms; consisting externally of two flat latticed sieve-plates and internally of a central subspherical to ellipsoidal latticed shell joined to the two plates by numerous radial beams and surrounded by three (in a few specimens, two), widely spaced, latticed, concentric rings which are perpendicular to the parallel plates (text-figure 12). External plates with subequal, subregularly to regularly arranged, circular to subcircular pores, without polygonal frames, generally four to five in number in the area between each ring; surface of plates either smooth or with a few thorns or short spines; margins of plates curve toward one another and are joined to form an outer marginal chamber in fully developed forms; pores of margin very small, circular, about as wide as the intervening bars; on the periphery of the margin of fully developed forms at one pole of the major axis of the complete test is a pylome, elliptical in section, which is surrounded by numerous, short, tooth-like spines. Internal concentric rings generally elliptical to subquadrangular in outline, with two to three rows of subequal, subcircular pores between the plates; rings separated by nearly equal distances. Radial beams, most of which are generally parallel to the plates, are variable in number (5-20 or more); some arise from the surface of the inner shell, others from the rings; few continue from the inner shell to the margin; no beams continue as free spines beyond the closed margin of fully developed forms; in most tests the beams terminate before reaching the closed margin. Most tests not fully developed, without complete margin.

Measurements; based on 30 specimens from stations 27, 34, 56, 60, and 71: maximum diameter of fully-developed tests (generally the axis perpendicular to the major axis of the internal rings) 148-246 µm, minor diameter of elliptical tests approximately 10-20 µm less than major diameter; major diameter of outermost ring 111-160 µm, of second ring 70-105 µm, of first ring 31-42 µm; minor diameter of outermost ring 91-149 µm, of second ring 64-97 µm, of first ring 21-31 µm; diameter of inner shell 9-l4 µm.

Remarks. This species has an internal structure similar to that of Sty1odictya validispina but differs from it in the constant number of three concentric rings and in the presence of a marginal chamber with small pores and a single pylome. It differs from the Ommatodiscus sp. group from the Gulf in number of concentric rings and in the absence of central, concentric, discoidal shells. The presence of a single pylome places this species within the genus Ommatodiscus Stöhr.
Photographic illustration of Carnevale's (1908, P1. 4, figs. 6-9) four species from the Miocene of Italy listed in the synonymy do not differ from the Gulf species. 0. reniformis appears to be a misshapen form, not separable as a species; the other three species were defined on the basis of the test outline, circular or elliptical, a feature subject to intraspecific variation in the Gulf species. The pores of Carnevale's species are slightly smaller than those of the Gulf specimens, but this is considered insignificant in relationship to the identical internal structure.

Distribution. This species is very rare in the Gulf. Its occurrence as far north as station 184 does indicate some tolerance for waters of slightly higher than average temperature and salinity. It is absent at stations 64, 115, 130, 151, and those north of station 184. It has a slightly greater frequency in the southern half of the Gulf; thus, it has greater affinity for the waters of the tropical Pacific than those of the Gulf.
Benson 1966











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