Page:Scientific results HMS Challenger vol 18 part 1.djvu/982

774 large as the two spines of the transverse axis. Eight tropical spines of similar form, but only half as large, and with a very large, extremely prominent, lamellated leaf-cross. Eight polar spines quite rudimentary, scarcely prominent.

Dimensions.—Length 0.36, breadth 0.2.

Habitat.—Central Pacific, Station 271, surface.

Definition.— with four equatorial spines of unequal size and form; both lateral spines (in the transverse axis) equal; but both principal spines (in the longitudinal axis) very different, the frontal larger than the caudal. Sixteen other spines much smaller (often the eight tropical larger than the eight rudimentary polar spines). No apophyses.

The genus Zygostaurus differs from all other Staurolonchida in the peculiar differentiation of the four equatorial spines; the two opposite spines of the hydrotomical or longitudinal axis being very different in size and form (the frontal spine forked, the caudal spine simple); whilst the two opposite spines of the geotomical or lateral axis (perpendicular to the former) are equal, forked, but different in shape from the former. Therefore the geometrical fundamental form of the body in this remarkable genus becomes "amphithect" or "bilateral" in the widest signification of this term (comp. my General Morphology, vol. i. pp. 480, 482). Of the three different dimensive axes one (the longitudinal) exhibits unequal poles, whilst the two other (the sagittal and lateral) axes exhibit equal poles.

1. Zygostaurus amphithectus, n. sp. (Pl. 131, fig. 7).

Frontal spine (c1) little different from the two lateral spines; each with two divergent curved horns, which are equal and of about the same length as the simple broad basal part. Caudal spine (c3) simple, spindle-shaped, about as long as the frontal spine. Tropical spines little smaller than the three former, and of the same symmetrical forked form. Eight polar spines much smaller, but also forked, with thin bristle-shaped branches.

Dimensions.—Length of the cross (longitudinal axis) 0.5, breadth (lateral axis) 0.4.

Habitat.—Central Pacific, Station 272, surface.

2. Zygostaurus longicornis, n. sp.

Frontal spine (c1) little different from the two lateral spines, each with two divergent curved horns, which are equal and of about the same length as the simple broad basal part. Caudal