Page:Scientific results HMS Challenger vol 18 part 1.djvu/957

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Definition.— with simple, four-edged, prismatic or pyramidal radial spines, without apophyses; their transverse section is square.

The genus Acanthonia comprises all those Astrolonchida (formerly united with Acanthometron) in which the simple spines either in their whole length or in the greatest part of it are four-edged, with square transverse section. They are sometimes more prismatic (with equal breadth), at other times more pyramidal (with decreasing breadth towards the distal apex). If Acanthometron be the common simple ancestral form of the Acanthonida, then the two-edged Zygacantha, and the four-edged Acanthonia may be regarded as two divergent main lines arising from it; the former leading to the Phractacanthida and Diporaspida, the latter leading to the Stauracanthida and Tessaraspida.

Definition.—Spines at the central base, without leaf-cross and without hollow pyramidal compartments, united by the triangular faces of their pyramidal bases, resting one upon another.

1. Acanthonia tetracopa, Haeckel (Pl. 129, figs. 9-11).

Acanthometra tetracopa, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. vii. figs. 3 to 5, Taf. xi. fig. 5.

Acanthometra tetracopa, Haeckel, 1862, Monogr. d. Radiol., p. 379, Taf. xviii. fig. 5.

Spines quadrangular prismatic, with prominent lamellar edges, of equal breadth in their whole length. Base pyramidal, without leaf-cross. Apex truncated, or with four short teeth (sometimes only two teeth). Central capsule opaque, brown or yellow.

Dimensions.—Length of the spines 0.15 to 0.2, breadth 0.01 to 0.012.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, many Stations, surface.

2. Acanthonia prismatica, n. sp.

Spines quadrangular prismatic, without prominent edges, with four plane lateral faces, of equal breadth throughout their whole length. Base pyramidal, without leaf-cross. Apex truncate, with square apical face.

Dimensions.—Length of the spines 0.1 to 0.16, breadth 0.005 to 0.008.

Habitat.—Antarctic Ocean, Station 154, surface.