S31

Actinomma medianum Nigrini

Actinomma medianum Nigrini, 1967, p. 27, pl. 2, figs. 2a, b.

DESCRIPTION

"Cortical shell a somewhat irregular sphere, thin-walled, with 9-12 subcircular to subangular pores on a half-equator.

"Medullary meshwork irregular, more delicate than, but otherwise similar to, that of Actinomma antarcticum, with an internal shell having 2.5-3 subcircular pores on a half-equator.

"Numerous radial beams extend from the outer surface of the medullary meshwork to the cortical shell and sometimes extend externally. Additional, more delicate spines, originating on the cortical surface, are present in some specimens. Occasionally, these spines support a delicate extracortical shell.

"... the species exhibits characters intermediate between A. antarcticum and A. arcadophorum. It may be distinguished from the former by its simply latticed cortical shell and more delicate medullary meshwork." (from Nigrini, 1967).

DIMENSIONS

"Diameter of cortical shell 164-227 microns; of medullary shell 81-100 microns. Inner medullary capsule approximately one-third the diameter of the medullary meshwork." (from Nigrini, 1967).

"cortical shell diameter, 221 +/- 32 microns; diameter of spongy medullary structure, 101 +/- 5 microns; (based on measurements of 11 specimens)" (from Sachs, 1973).

REMARKS

1. Benson's (1966) description and dimensions of this species (Diploplegma banzare Riedel in Benson, p. 134) are consistent with the above.

2. "Nigrini (1967) described this species as having characteristics intermediate between Actinomma antarcticum (Haeckel) and Actinomma arcadophorum Haeckel. All three species were encountered in this study. Nigrini's differentiation between these species can be extended to the South Atlantic. A. antarctica usually has two heavy cortical shells, where A. medianum has only one cortical shell. A. arcadophorum is distinguished in the study area by its large, delicate cortical shell containing many small pores." (from Morley, 1977).

Plate 3, figures 5, 6

S32

RECENT DISTRIBUTION

1. Benson, 1966 (Diploplegma banzare); "This species has a much greater occurrence in the northern Gulf than in the central or southern parts [which] may reflect its tolerance of higher salinity and temperature.

2. Nigrini, 1967, fig. 11; "Distribution of A. medianum in the Indian Ocean is also intermediate between that of A. antarcticum and A. arcadophorum. A. antarcticum is found in cold (Antarctic) water, A. medianum in cool (middle latitude) water and A. arcadophorum in warm (low latitude) water." "Indian Ocean occurrences - A. medianum is almost completely absent from low latitudes, but forms 6 percent to 12 percent of the described population in middle latitude samples. The species is present in high latitudes, but is far out-numbered by A. antarcticum. A. medianum appears to be a reliable and potentially useful member of the middle latitude assemblage."

3. Nigrini, 1970, fig. 9; belongs to a transitional assemblage derived by recurrent group analysis of North Pacific samples, although it does occur rarely in tropical sediments."

4. Sachs, 1973, Code 8S + 22S, figs. 2A; "Occurs to 7% of total fauna, and exhibits strong Southern (Transitional Factor) preferences." Southern is south of about 45 degrees N.

5. Molina-Cruz, 1975, Code S3; counted together with A. arcadophorum and used in factor analysis of southeast Pacific assemblages; cf. Appendix 10 for percent S3 at each station.

6. Robertson, 1975; "The medullary shell of A. medianum is frequently encountered and is included in the counts of this species... This species loads most heavily in factor 3 (transitional). At the present it occurs most frequently between 35 degrees and 43 degrees N. At 18,000 YBP higher abundances are found farther to the south."

7. Morley, 1977; Fig. I-7; "This species loads highest is factor 3 (gyre margin). Although it appears to have a somewhat cosmopolitan distribution, this species is most common in samples between 45 degrees and 52 degrees S at present. Lower abundances in most areas are characteristic of this species' distribution 18,000 YBP."