Definition.—Quadrilonchida with four equatorial spines of equal size and form, which are much larger than the sixteen other spines. Eight tropical and eight polar spines nearly equal. No apophyses.
The genus Acanthostaurus is the most simple and primitive form of the Quadrilonchida, and the common ancestral genus of this family; it is at the same time its most common and widely distributed form. Some species appear in astonishing numbers in different seas. It has been derived from Acanthometron by stronger development of the four equatorial spines, which are all of equal size and much larger than the sixteen others.
Subgenus 1. Acostaurus, Haeckel.
Definition.—All twenty spines separated, in the centre united only by the triangular faces or the meeting leaf-shaped edges of their pyramidal bases.
1. Acanthostaurus aequatorialis, n. sp.
Spines cylindrical, of nearly equal breadth throughout their whole length. Apex simple conical. Base with a large leaf-cross. Four equatorial spines of the same form as the sixteen others, but much longer and about three times as broad.
Dimensions.—Length of the four major spines 0.2 to 0.25, breadth 0.008; length of the sixteen minor spines 0.01 to 0.15, breadth 0.003.
Habitat.—Equatorial Pacific, Station 271, surface.
2. Acanthostaurus bipennis, n. sp.
Spines linear, sword-shaped, strongly compressed, two edged; of nearly equal breadth in their whole length. Apex bifid. Base with a small leaf-cross. Four equatorial spines of the same form as the sixteen others, but twice as long and as broad.