Habitat.—North Pacific, Station 236, surface.
3. Zygostephanium constrictum, n. sp.
Frontal ring violin-shaped, concave on the upper and lower margins, with a deep sagittal constriction. Sagittal ring ovate, about two-thirds as high as the frontal ring. Both rings covered with numerous branched irregular spines, which partly anastomose along the edges of the rings, and produce small irregular polygonal pores.
Dimensions.—Height of the frontal ring 0.12, breadth 0.18.
Habitat.—Central Pacific, Station 272, depth 2600 fathoms.
Subfamily 2. Acanthodesmida, Haeckel, 1881, Prodromus, p. 445.
Definition.—Coronida with five large gates (four lateral and one basal). Skeleton composed of three rings, perpendicular one to another, two of which are vertical and incomplete (the primary or lateral, and the secondary or frontal), the third is horizontal and complete (the tertiary or basal ring).
Genus 416. Coronidium,[[35]] Haeckel, 1881, Prodromus, p. 445.
Definition.—Coronida with five large simple gates (four lateral and one basal). Skeleton composed of two incomplete meridional rings and one complete basal ring, without lattice-work.
The genus Coronidium and the following, nearly allied Acanthodesmia, form together the peculiar subfamily of Acanthodesmida—not in the wider sense in which I first founded this group (1862, Monogr. d. Radiol., p. 265), but in the restricted sense, which is exactly defined in my Prodromus (1881, p. 445). According to this definition, the shell is composed of three different rings, perpendicular to one another; only one of these is complete, the simple horizontal basal ring; the two others are incomplete and vertical (the primary or sagittal and the secondary or frontal ring). Therefore there remain constantly between the three rings five characteristic large openings or gates; four of these are lateral (between the halves of the two meridional rings), the fifth is basal, enclosed by the horizontal basal ring. The longest known type of this subfamily is Acanthodesmia vinculata, the five characteristic gates of which are clearly distinguished by its discoverer, Johannes Müller ("Das Gehäuse besteht nur aus den Leisten zwischen fünf grossen Lücken"). The Acanthodesmida may be derived from the Eucoronida by reduction of the basilar rod of the sagittal ring. If in Eucoronis this basal rod be lost, Coronidium arises.