S75

Amphirhopalum ypsilon Haeckel

Amphirhopalum ypsilon Haeckel, 1887, p. 522; Nigrini, 1967, p. 35, pl. 3, figs. 3a-3d.

DESCRIPTION "Shell with 2 opposite, chambered arms, one of which is forked distally. Arms arise from a central structure composed of 2 inner spherical shells and an outer oblate spheroidal shell, all quite smooth and connected by numerous, discontinuous, radial beams. In addition, there is an outer ring of mesh in the plane of the shell which is normally oriented perpendicular to the microscope axis. This orientation makes the central structure appear as a 'central, concentrically annulated disc' (cf. Haeckel, 1887, p. 516, Subfamily Euchitonida), because the spheroidal shell has its minor axis along the microscope axis and the external ring, or rings, are in the plane of the slide.

"In cross section arms are elliptical with their shortest dimension normally oriented along the microscope axis. Unforked arm is narrow proximally, expands distally to a maximum breadth about two-thirds of the way along its length, then decreases slightly in breadth to a blunt termination. Usually 4-9 distinct chambers, convex distally, can be seen; however, shell may become spongy over the distal one-third of the arm. Similarly, the forked arm expands distally and branches where its breadth reaches a maximum. Usually 5-9 distinct chambers, including those on the branches, convex distally; chambers on branches sometimes obscured by spongy meshwork.

"Internal spines form a basic framework which is covered by a lattice of small circular to subcircular pores.

"In some specimens a patagium is present around the central structure and arms, sometimes with 4 or 5 chambered rows, concave inwards; sometimes simply a spongy mass. It seems probable that a complete patagium (i.e., on a fully developed specimen) might surround the whole basic shell structure, but in all specimens examined the patagium had developed only between the 2 main arms and around the central structure. Patagium generally more delicate than the main shell." (Nigrini, 1967).

"Specimens from the upper parts of the cores examined [Late Quaternary] average four or five proximal chambers on the forked arm before it bifurcates. Lower down in the cores [Early Quaternary] this number decreases, and forms with two or three (sometimes one) such chambers predominate. The decrease coincides approximately with an increase in abundance." (Nigrini, 1971).

Plate 10, figures 1a-e

S76

DIMENSIONS "Total length 236-307 microns. Radius of simple arm 119-155 microns; of forked arm 119-155 microns. Maximum breadth of simple arm 63-119 microns; of branches on forked arm 36-63 microns." (Nigrini, 1967).

REMARKS

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

2. "Dumitrica (1973, p. 835) described a species called Amphirhopalum wirchowii (Haeckel). There is a minor spelling error and the name should be A. virchowii. This species is very likely the ancestor of A. ypsilon, and differs from it in having smoother, more distinct and more rounded chambers. It has previously been observed by the present author in Upper Pliocene sediments and there is a transition from A. virchowii to A. ypsilon near the Plio-Pleistocene boundary. A. ypsilon then evolves in the manner described by Nigrini (1971, p. 447). The specimens figured by Dumitrica (1973, pl. 11, fig. 6) and by Haeckel (1862, pl. 30, fig. 4) are good examples of A. virchowii. The arms are somewhat narrower than those illustrated by Haeckel (1862, pl. 30, figs. 1, 2). Specimens figured by Nigrini (1971, pl. 1, figs. 7b, 7c) and referred to by Dumitrica (1973) are good examples of early A. ypsilon. A. virchowii is a convenient name for the species in question, but without examining type material, it is difficult to say whether Haeckel was looking at the Pliocene form or early A. ypsilon." (from Nigrini, 1974).

3. Benson's (1966) description and dimensions of this species (Amphicraspedum wyvilleanum (Haeckel) in Benson, p. 221) are consistent with the above.

4. Note spelling error in Moore (1974) and Molina-Cruz (1975).

S77

RECENT DISTRIBUTION

1. Benson, 1966 (Amphicraspedium wyvilleanum); "This species is cosmopolitan in the Gulf [of California], being absent only at stations 203, and 214. It is rare at all stations but is present in slightly greater numbers at stations located in the axial portion of the Gulf. Its average frequency is slightly greater in the southern half of the Gulf; therefore, it is a more nearly oceanic species. It undergoes no marked increase at stations located within regions of upwelling."

2. Nigrini, 1967, fig. 16; "Indian Ocean occurrences - A. ypsilon is sparsely distributed in low latitudes and is absent from samples south of 35 degrees S. Though rather rare in Recent sediments, it appears to be a useful member of the low latitude assemblage."

3. Nigrini, 1968, text-fig. 13; "Few in the regions of the North and South Equatorial Currents, but decreasing in abundance westward and in the region of the Equatorial Countercurrent."

4. Nigrini, 1970, fig. 14; 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.

5. Molina-Cruz, 1975, Code S9; not used in factor analysis of southeast Pacific samples; cf. Appendix 10 for percent S9 at each station.