Page:Scientific results HMS Challenger vol 18 part 1.djvu/976

768 less compressed from both poles of the spineless axis, lenticular or discoidal, sometimes square. It is enveloped by a voluminous calymma constantly bearing coronals of "Myophrisca" (compare p. 724).

Definition.— with simple radial spines, without apophyses.

Definition.— 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.