Habitat.—Fossil in Barbados.
8. Podocoronis polypodiscus, n. sp.
Basal ring elliptical, with fifteen to eighteen conical, curved, descending feet, half as long as the ovate sagittal ring. Frontal ring kidney-shaped, one and a half times as broad as long. All the rings armed with short conical spines.
Dimensions.—Height of the frontal ring 0.12, breadth 0.18.
Habitat.—Central Pacific, Station 273, depth 2350 fathoms.
Subfamily 4. Trissocyclida, Haeckel, 1881, Prodromus, p. 446.
Definition.—Coronida with eight large gates (four superior lateral and four inferior basal). Skeleton composed of three complete rings, perpendicular to one another; two of which are vertical (the primary or sagittal and the secondary or frontal ring), the third is horizontal (the tertiary or basal ring).
Genus 421. Tristephanium,[[40]] Haeckel, 1881, Prodromus, p. 445.
Definition.—Coronida with eight large, simple gates of different sizes; the four upper or lateral gates larger than the four lower or basal gates. Skeleton composed of three simple complete rings, perpendicular to one another.
The genus Tristephanium, and the three following genera derived from it, represent together the interesting subfamily of Trissocyclida (Prodromus, 1881, p. 446). These differ from all other Coronida in the possession of three complete rings, perpendicular to one another. The first of these is the vertical sagittal ring, the second the vertical frontal ring, and the third the horizontal basal ring. Since these three rings lie in the three dimensive planes, they are perpendicular to one another; and between them remain eight large open gates. Originally the four upper or lateral gates (corresponding to those of Zygostephanus) are much larger, the four lower or basal gates (corresponding to those of Semantrum) much smaller; but afterwards the latter may reach the size of the former, so that the basal ring becomes equatorial (in Trissocircus and Trissocyclus). The common ancestral form of the Trissocyclida (Tristephanium) may be derived directly either from Semantrum (by complete development of the frontal ring) or from Eucoronis (by complete development of four basal gates).