Page:Scientific results HMS Challenger vol 18 part 2.djvu/32

Synopsis of the Genera of Plagonida.

I. Subfamily Triplagida. Three radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Three spines lying in one horizontal plane,			384. Triplagia.
		Three spines corresponding to the edges of a flat pyramid,			385. Plagiacantha.
II. Subfamily Tetraplagida. Four radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Four spines arising from one common central point.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	386. Tetraplagia.
				One apical spine opposed to three basal spines,	387. Plagoniscus.
		Four spines arising in two pairs from the poles of a common central rod.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	388. Plagonidium.
				One apical spine opposed to three basal spines,	389. Plagiocarpa.
III. Subfamily Hexaplagida. Six radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Six spines arising from one common central point,			390. Hexaplagia.
		Six spines arising in two opposite groups from the poles of a common central rod,			391. Plagonium.
IV. Subfamily Polyplagida. Numerous radial spines.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod),			392. Polyplagia.

Subfamily 1. Triplagida, Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with three radial spines.

Genus 384. Triplagia,^[1] Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus Triplagia and the following closely allied Plagiacantha may be regarded as the simplest and most primitive forms of the Plectoidea, perhaps as the common ancestral stock of this suborder. The skeleton is composed of three simple or branched radial spines, arising from one common central point. These three spines in Triplagia lie in one and the same plane, whilst in Plagiacantha they lie in different planes. Therefore the former exhibits the simplest type of the triradial structure, common to the majority of Nassellaria.

1. Triplagia = Triangular or three-radial; τριπλάγιος.