Page:Scientific results HMS Challenger vol 18 part 1.djvu/761

Rh thin disk it nevertheless reaches unusual dimensions, its diameter in some species being more than a millimetre.

Definition.—Posterior arms simple, undivided; anterior arms lobated or cleft, with one or more incisions at the distal end.

1. Myelastrum medullare, n. sp. (Pl. 47, fig. 13).

Anterior arms bifid, nearly square, with a shallow incision at their broad truncated end. Posterior arms somewhat smaller, nearly triangular, with simple blunt ends. Sagittal constriction three-fourths as large as the transverse one. Surface smooth.

Dimensions.—Radius of the anterior arms 0.36, of the posterior 0.3; longitudinal constriction 0.24, transverse 0.36.

Habitat.—Pacific, central area, Station 271, surface.

2. Myelastrum spinale, n. sp.

Anterior arms bifid, twice as long as broad, with a deep incision at their truncated end. Posterior arms slender, half as large, with simple blunt ends. Sagittal constriction one and a half times as large as the transverse. Surface spiny.

Dimensions.—Radius of the anterior arms 0.4, of the posterior 0.2; longitudinal constriction 0.3, transverse 0.2.

Habitat.—North Pacific, Station 248, surface.

3. Myelastrum heteropterum, n. sp. (Pl. 47, fig. 8).

Anterior arms trifid, about as long as broad, with two incisions at their broad truncated end. Posterior arms about half as large, simple, with rounded blunt ends. Sagittal constriction two-thirds as large as the transverse. Surface bristly.

Dimensions.—Radius of the anterior arms 0.65, of the posterior 0.035; longitudinal constriction 0.4, transverse 0.6.

Habitat.—South Pacific, Station 291, surface.

Definition.—All four arms (posterior as well as anterior) lobated or cleft.

4. Myelastrum octocorne, n. sp. (Pl. 47, fig. 12).

All four arms with a deep incision in their distal half; anterior arms broader, but shorter than the posterior; in the anterior arms the two lobes are of the same size, in the posterior arms the