S35

Actinomma leptodermum (Jorgensen)

Echinomma leptodermum Jorgensen, 1900, p. 57; 1905, p. 116, pl. 8, figs. 33a-c

Actinomma sp., Benson, 1966, p. 164 (partim.), pl. 5, fig. 6 (only)

DESCRIPTION AND DIMENSIONS

"The outer ball thin-walled (the walls broader than they are thick). The pores polygonally roundish oval, very uneven in size, 7-25 microns, with intermediate walls (2-4 microns broad), which are much broader towards the corners (lumen rounded off).

"The middle shell moderately thick (the intermediate walls being as thick as they are wide, about 1 1/2 microns), rather angular and irregular, a little larger than in Hexacontium enthacanthum; diameter about 40 microns. The pores are somewhat uneven, roundish, 4-7 microns. The intermediate walls solid, not particularly broader in the corners.

"It is difficult to see the inmost shell, which possesses solid beams (about equal in thickness to those of the middle shell), but rather few polygonal, mostly pentagonal or hexagonal pores, about 8 microns. The diameter of the inmost shell about 15 microns (or a little more).

"About 15 main spines, about equally broad inside as outside of the outmost shell, not long. They seldom protrude farther than to a length equal to the distance between the two outer shells, often less, and vary in development. Between the two inner shells, the radial spines are very narrow and in fact hardly wider than the beams of the inmost shell.

"The byspines on the outside shell are in appearance like the main spines, but not radially lengthened inwards, with a wide base on the outer shell (like the main spines) and very unevenly developed in size, although generally protruding less than the main spines. Variable in number; although, as a rule, not many, far from being developed in all the corners, only here and there.

"The number of the main spines is variable often only about 10, though oftenest about 15. They are 3-edged....there are forms without outer shells, but there is generally a trace of these in transverse processes on the main spines. These may, however, also be entirely absent...

"This species also varies a good deal. When the outside shell is thin-walled, the pores and intermediate walls are of a more uneven size. The byspines are in such cases slightly developed or (as yet) wanting. "It is likely that thses divergences may be accounted for by a difference in age. A more important difference is the number of main spines, which seems to be able to vary from 10 to 16." (from Jorgensen, 1905).

Plate 3, figure 7

S36

RECENT DISTRIBUTION

1. Benson, 1966 (Actinomma sp.); rare or absent in all Gulf of California stations.

2. Sachs, 1973, Code 7S (Actinomma sp.); "Occurs preferentially in Southern samples, with maximum frequency 3%. Anomalous indication (2%) at Northern count [024] due to an uncorrected error: Specimens of another species, probablyHexacontium enthacanthum were incorrectly included with the present species, although the former has significantly fewer spines (5-6 vs. 10-12)." Southern is south of about 45 degrees N.

3. Molina-Cruz, 1975, Code S4 (Actinomma spp.); not used in factor analysis of southeast Pacific assemblages; cf. Appendix 10 for percent S4 at each station.

4. Robertson, 1975 (Actinomma spp.); "This group loads most heavily in factor 3 (transitional). At the present it occurs most frequently in the northwest Pacific north of 35 degrees N. At 18,000 Y.B.P. there is no discernible shift in its distribution."

5. Morley, 1977 (Actinomma spp.1); "This taxonomic group loads highest in factor 3 (gyre margin). At present it occurs most frequently in samples located between 25 degrees and 45 degrees S. At 18,000 Y.B.P. its over-all abundance decreased and the region of its maximum concentration shifted 5 degrees to 10 degrees northward."

Note: There is considerable variation between authors concerning the limits of this species. Comparison of distributional data should be made with caution.