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

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).

Synopsis of the Genera of Quadrilonchida.

II. Subfamily Acanthostaurida All twenty spines simple, without lateral apophyses (sometimes forked, but neither branched nor latticed).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Four equatorial spines of equal size and form.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Eight tropical and eight polar spines nearly equal,	334. Acanthostaurus.
				Eight tropical and eight polar spines very different,	335. Belonostaurus.
		Four equatorial spines of very different size or form (the two lateral constantly equal).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Two principal spines of equal size and form,	336. Lonchostaurus.
				Two principal spines (frontal and caudal) very different,	337. Zyostaurus.
II. Subfamily Lithopterida. Either all twenty spines or a part of them provided with two opposite lateral branches or apophyses.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Apophyses simple, neither branched nor latticed,			338. Quadrilonche.
		Apophyses branched or pinnate, but not latticed,			339. Xiphoptera.
		Apophyses latticed, with fenestrated network,			340. Lithoptera.

Subfamily 1. Acanthostaurida, Haeckel, 1881, Prodromus, p. 466.

Definition.—Quadrilonchida with simple radial spines, without apophyses.

Genus 334. Acanthostaurus,^[1] Haeckel, 1862, Monogr. d. Radiol., p. 395.

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.

1. Acanthostaurus = Spine-cross; ἄκανθα, σταυρός.