Definition.—Porodiscida with four forked, spongy, or chambered arms, without a patagium; shell bilateral, with paired different arms; two equal anterior arms of different shape from the two equal posterior arms.
The genus Myelastrum differs from the foregoing Dicranastrum, the ancestral form, by the twofold differentiation of the four arms. Whilst in the latter all four arms are equal, separated by equal angles, here the two anterior arms are constantly different from the two posterior. The lateral angles between the two arm-pairs are equal, while the posterior and the anterior angle (between the two arms of each pair) are more or less different. The shell assumes, therefore, a very characteristic bilateral form, similar to the "quadricorn cross" of the grey central substance in the transverse section of the human medulla spinalis. Though the spongy shell is commonly a most delicate and thin disk it nevertheless reaches unusual dimensions, its diameter in some species being more than a millimetre.
Subgenus 1. Myelastrella, Haeckel.
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.