Sl07

Porodiscus sp.A

Ommatodiscus sp. Benson, p. 210, pl. 10, fig. 3 (only).

Stylodictya sp. Petrushevskaya, 1967, p. 32, fig.18,I, II.

DESCRIPTION

"Skeleton in form of disk thickened in central part and therefore opaque. Second and third rings are first to be seen distinctly. Usually chambers often disposed in spiral turns rather than in concentric rings... Width of chambers not increasing to periphery of disk. Pores irregular; usually three pores lying on width of chamber. Pores of approximately uniform dimensions on central and peripheral chambers....Radial needles seen extending from margin of shell; owing to apophyses of these needles the following chambers form....

"In the Pacific Ocean sector [of the Antarctic Ocean]....several other forms are encountered. Differ from above usually in greater number of rings (7-8) observed), and therefore in the greater size of the disk. Disposition of pores somewhat more regular, pores are larger, and only 2.5 of them are located on the width of one chamber (i.e., one pore usually lies on the seam between the chambers of neighboring rings). Chambers of various rings, as in the typical variety described above, of uniform width about 10 , not expanding toward periphery of the disk. The margins of the disk in all specimens were unfortunately broken down". (translated from Petrusevskaya, 1967).

DIMENSIONS "diameter of central opaque part of shell 25-40mu, width of one row of chambers about 10mu, diameter of shell with 4-5 marked turns 110-125mu". (translated from Petrushevskaya, 1967).

RECENT DISTRIBUTION:

1. Sachs, 1973, Code 121 (Ommatodiscus sp.): Sachs apparently follows Benson's (1966) concept of this species which includes a number of different forms. Therefore, his distributional information may be inaccurate for the species here described, i.e., "This form strongly favors southern (Transitional) waters".

Plate 14, figures 1, 2a,b

Sl08

2. Lozano, 1974, Fig. IV-II (Ommatodiscus sp.): "There are several closely related species which have been classified in various ways by different authors and are difficult to distinguish from one another in permanent slides. We restrict our counts to three variants which are included by Benson (1966) within his definition of Ommatodiscus sp. characterized by having concentric, latticed, discoidal shells in the central region of the text so that inside view of the central region of the test is biconvex. We limited our counts to those forms which in plain view are seen as made of concentric or spiral rings, the width of the rings increasing very little outwardly. In some specimens the first two or three rings cannot be resolved....Other specimens have the first two concentric rings followed by a spire of two or three whorls....In other forms all rings can be distinguished and are concentric from the first to the last one....Other specimens in which the central part can be resolved have a continuous spiral shape....We did not include in our counts other types of Ommatodiscus which have irregular rings. We differentiated Ommatodiscus sp. from Stylodictya validispina Jorgensen (Benson, 1966) because in the latter one the first and second and sometimes the third ring have a quadrate outline and the second ring consists of four separated chambers....However, it is possible that we may have included in our counts some of the latter in which the quadrate outline of the inner rings was not apparent.

"It is present at all locations within the studied area constituting less than 0.1 percent only in core V29-87. It is more abundant under subtropical waters where it generally accounts for over 2.5 percent of the total radiolarian fauna reaching a maximum in cores RC13-244, V24-202 (7.1 percent), and V18-188 (7.0 percent). Under subantarctic waters it generally accounts for 0.5 to 2.5 percent of the total Radiolaria. Under Antarctic waters it is found in abundances of 0.1 to 0.9 percent. The maximum values (7 percent) correspond to sea surface temperatures of 18° to 21°C in February and about 15°C in August. Present in proportions over 2 percent only under waters warmer than 12° in February and 7°C in August. It consititues less than 1 percent of the total Radiolaria under waters with temperatures below 6°C in February and 2°C in August."

3. Molina-Cruz, 1975, Code S45 (Ommatodiscus sp. A); used in factor analysis of southeast Pacific assemblages; cf. Appendix 10 for Percent S45 at each station.

4. Robertson, 1975; counted together with Porodiscus (?) sp. B. "This species loads heavily in factor 2 (subtropical). At present this species is most abundant to the south. At 18,000 YBP there is a marked southern shift of the regions of highest abundance "