Amphirhopalum ypsilon Haeckel, 1887, p.522; Nigrini, 1967,p.35, pl.3, figs.3a-3d (with synonymy)
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 that 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 that 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).
Total length 236-307 µm. Radius of simple arm 119-155 µm; of forked arm 119-155 µm. Maximum breadth of simple arm 63-119 µm; of branches on forked arm 36-63 µm (Nigrini, 1967).
This species is common in tropical sediments from all oceans. Its morphotypic first appearance is in the early part of the Spongaster pentas Zone and it is extant. Rare at the lower end of its range, but easily recognizable, even from fragmentary specimens. Morphotypic first appearance is synchronous.