N41

Pterocanium praetextum praetextum (Ehrenberg)

Lychnocanium praetextum Ehrenberg, 1872a, p. 316

Pterocanium praetextum (Ehrenberg), Haeckel, 1887, p. 1330, pl. 73, fig. 6

Ptercanium praetextum praetextum (Ehrenberg), Nigrini, 1967, p. 68, pl. 7, fig. 1

DESCRIPTION

"Cephalis subspherical, with closely-spaced pittings probably representing infilled pores. Apical horn usually cylindro-conical, occasionally angular, as long to twice as long as cephalis breadth. Thorax basically hemispherical, but with shape greatly modified by swellings between the 3 thoracic ribs, resulting in resemblance to a 3-cornered biretta in which the thoracic ribs are situated in depressions across the faces. Thoracic pores subcircular to subangular, separated by relatively thin bars. Feet 3-bladed, proximally fenestrated, curved with convexity outward near thorax, becoming almost straight distally, sub-parallel or slightly divergent, tapering to a point. Abdomen thin-walled, varying in state of development, having small subcircular to angular pores over most of its area, with a row of larger pores adjacent to the feet and in some individuals also along edge of thorax... This species differs from others of the genus in the biretta-shaped thorax and generally straight, almost parallel feet." (from Riedel, 1957).

"Specimens examined by the author are identical with those described by Riedel (1957). A feature not previously described, however, is that inside the cephalis both apical and vertical spines are free of the shell wall. Externally, the apical spine forms a strong cylindroconical horn; the vertical spine, a short, very much more delicate horn. On a few specimens additional by-spines occur of the cephalis and upper thorax." (from Nigrini, 1967).

DIMENSIONS "Length of apical horn 30-50 microns, of cephalis 14-20 microns, of thorax 50-60 microns, of abdomen 5-120 microns, of feet 75-165 microns. Breadth of cephalis 22-30 microns, of thorax 70-90 microns." (from Riedel, 1957).

REMARKS 1. For a more complete synonymy see Nigrini, 1967.

2. Benson's (1966) description and dimensions of this subspecies (p. 408) are consistent with the above except for a less well-developed abdomen. .

Plate 23, figure 2

N42

3. Molina-Cruz (1975) and Lozano (1974) combined this subspecies with P. praetextum eucolpum in their counts; Lozano notes that "most of the specimens in our area (i.e., Antarctic, Atlantic and Indian sectors) belong to the subspecies P. praetextum eucolpum but in the northernmost samples in the Atlantic, though it is not abundant, P. praetextum praetextum is the predominant subspecies and in some of them is the only subspecies present. In the slide prepared from core V27-199, there are 10 specimens, all of them P. praetextum praetextum. Some specimens in samples from the subtropical Atlantic (i.e., V18-166) have a thin three-bladed apical horn but the thorax is like the one of praetextum, no abdomen was observed (specimens are not well preserved), and the feet may be as short as 40 microns."

RECENT DISTRIBUTION

1. Benson, 1966; "This species is very rare in the Gulf [of California]."

2. Nigrini, 1967, fig. 35; Indian Ocean occurrences - P. praetextum praetextum is abundant (3% to 14% of the described population) in low latitudes, but does not occur in samples from south of 35 degrees S. It appears to be a reliable and potentially useful member of the low latitude assemblage.

3. Nigrini, 1970, fig. 24; belongs to a tropical assemblage derived by recurrent group analysis of North Pacific samples, but the species does range as far north as 40 degrees N in the western Pacific.

4. Lozano, 1974, Fig. IV-5; counted together with P. praetextum eucolpum; "It is found in a few samples in the eastern Atlantic and in the Indian Ocean constituting always less than 0.7 of the total radiolarian fauna. It is found in relatively high percentages (0.2 to 0.6) in the Crozet basin indicating southward (occasional ?) penetration of sub-tropical waters. It is also present in core RC11-120 at the junction of the Kerguelen Plateau and the Mid Ocean Indian Ridge. The presence of the subspecies P. praetextum praetextum (see discussion under Taxonomic notes section) which is found only north of 33 degrees S in the Indian Ocean (Nigrini, 1967) is in accord with the hypothesis proposed by Goll and Bjorklund (1974) that microplankton are seasonally introduced into the Atlantic Ocean during the summer months when the warm waters of the Agulhas current penetrate deeper into the Atlantic Ocean. The influence of the Agulhas current is suggested also by the distribution pattern of other species."

5. Molina-Cruz, 1975, Code N27; counted together with P. pratextum eucolpum and used in factor analysis of southeast Pacific assemblages; cf. Appendix 10 for percent N27 at each station.