The genus Hylaspis exhibits the same structure of the shell as the nearly allied ancestral genus Icosaspis, and differs from it only in the development of by-spines. Some species of these two genera exhibit the highest degree of complication in the structure of the shell seen among the Dorataspida.
1. Hylaspis serrulata, n. sp. (Pl. [135], fig. 1).
Parmal meshes four hundred to five hundred; in the centre of each plate a cross of four very large pentagonal or roundish aspinal pores, and around this a circle of sixteen to twenty much smaller irregular, polygonal, coronal pores; the latter of about the same size as the sutural pores. On each condyle one thin zigzag-shaped by-spine, nearly as long as the radius. Twenty radial spines very long, quadrangular, prismatic; on the inside thinner and smooth, on the outside thickened, and armed with four rows of recurved teeth, serrated.
Dimensions.—Diameter of the shell 0.18, aspinal spines 0.02, other pores 0.002 to 0.01, bars 0.003.
Habitat.—South Atlantic, Station 326, surface.
2. Hylaspis coronata, n. sp.
Parmal meshes five hundred to six hundred; in the centre of each plate a cross of four long rectangular aspinal pores, between these four larger egg-shaped angular pores; around this rosette a circle of sixteen to twenty much smaller, irregular, roundish, coronal pores; the latter about of the same size as the sutural pores. On each condyle one bearded by-spine about one-third or one-fourth as long as the radius. Twenty radial spines, very long, smooth, quadrangular, prismatic.
Dimensions.—Diameter of the shell 0.25, aspinal pores 0.02, other pores 0.005 to 0.01, bars 0.004.
Habitat.—Central Pacific, Station 271, surface.
3. Hylaspis barbata, n. sp.