Subfamily 5. Euchitonida, Haeckel.
Definition.—Porodiscida with two or more (commonly three or four) radial chambered or spongy arms on the margin of the concentrically annulated disk, situated in its equatorial plane (with or without a connecting patagium between the arms).
Genus 223. Amphibrachium,[[262]] Haeckel, 1881, Prodromus, p. 460.
Definition.—Porodiscida with two simple, undivided, chambered arms, opposite in one axis, without a patagium.
The genus Amphibrachium opens the long series of the Euchitonida, or of those Porodiscida which bear on the margin of the circular central disk a certain number of chambered arms, composed of a series of chambers which are separated by transverse septa. The first group or tribe of this subfamily is formed by the Amphibrachida, in which the disk bears only two arms opposite on the poles of one axis. The simplest form of these is Amphibrachium, in which both arms are simple, equal, and without a patagium or spongy connecticulum.
Subgenus 1. Amphibrachella, Haeckel.
Definition.—Both arms equal, of the same form and size, blunt at the distal end, without a terminal spine.
1. Amphibrachium sponguroides, n. sp.
Both opposite arms of the same form and size, nearly cylindrical, three times as long as broad, with six to eight transverse septa or joints, at the distal end rounded, blunt, without a terminal spine.
Dimensions.—Radius of each arm 0.22, breadth 0.065.