N63

Eucyrtidium hexagonatum Haeckel

Eucyrtidium hexagonatum Haeckel, 1887, p. 1489, pl. 80, fig. 11; Nigrini, 1967, p. 83, pl. 8, figs. 4a, b

DESCRIPTION

"Cephalis simple, spherical, with numerous subcircular pores and an erect, or sometimes oblique, needle-like apical horn supported by 3 membranous buttresses; length of horn 1 or 2 times diameter of cephalis. Apical spine free. Cephalis usually depressed into thorax; median bar and vertical spine form a V. Primary lateral and dorsal spines continue as ribs in the thoracic wall, giving the segment a 3-lobed appearance from some angles, and then continue on in the abdominal wall for almost its entire length. Rarely, 1 or more of these ribs becomes external, forming small latticed wings.

"Thorax small, inflated annular with rough surface and rather thick wall. Thoracic pores subcircular, usually irregularly arranged, but sometimes in longitudinal rows. Lumbar stricture distinct.

"Abdomen and up to 5 post-abdominal segments, thin-walled, smooth; they expand distally, reaching a maximum breadth at about the second or third post-abdominal segment, and then constrict slightly. Most specimens are broken off at this point, and thus appear to have a wide mouth, very slightly constricted; however, in relatively rare complete specimens, narrowing continues and a short cylindrical pored terminal tube is formed. Pores circular to subcircular, arranged in longitudinal rows, but may be irregular in the final 2 or 3 segments." (from Nigrini, 1967)

DIMENSIONS "Total length (excluding apical horn) 146-209 microns. Diameter of cephalis 9-18 microns. Length of thorax 9-18 microns. Breadth of thorax 27-36 microns; maximum breadth 72-128 microns." (from Nigrini, 1967)

REMARKS

1. Petrushevskaya (1971) placed this species in synonymy with Eucyrtidium dictyopodium (Haeckel). However, the 3 shovel-shaped feet described and illustrated by Haeckel for that species have never been observed on E. hexagonatum and for that reason the synonymy is rejected herein.

2. Benson's (1966) description and dimensions of this species (Eusyringium siphonostoma Haeckel in Benson, p. 498) are generally consistent with the above although he does seem to have some longer specimens.

Plate 24, figures 4a,b

N64

RECENT DISTRIBUTION

1. Benson, 1966 (Eusyringium siphonostoma); "This species is one of the most abundant nasselline species in the Gulf [of California] ... suggests its tolerance of waters with higher than average temperature and salinity."

2. Nigrini, 1967, fig. 44; "Indian Ocean occurrences - E. hexagonatum is rather sparsely distributed in low latitudes (1% to 4% of the described population), and is practically absent in the middle latitudes. Though rather rare in Recent sediments, it appears to be a useful member of the low latitude assemblage."

3. Nigrini, 1970, fig. 29; belongs to a tropical assemblage derived by recurrent group analysis of North Pacific sediments, but the species does range as far north as 40 degrees N in the western Pacific.