5. _Eucoronis cervicornis_, n. sp. Frontal ring kidney-shaped, in the upper half convex, in the lower concave, with sagittal constriction. Sagittal ring ovate, two-thirds as high as the frontal ring. Both rings armed with numerous stout spines, irregularly branched like a deer's antler. (Very similar to _Coronidium cervicorne_, Pl. 82, fig. 1, which I formerly confounded with it, but differing in the complete sagittal ring, which in the latter form is incomplete and has lost its basal part.) _Dimensions._--Height of the frontal ring 0.12, breadth 0.2. _Habitat._--Central Pacific, Station 264, surface. 6. _Eucoronis arborescens_, n. sp. Frontal ring kidney-shaped, with a deep sagittal constriction in the concave basal part. Sagittal ring ovate, half as high as the frontal ring. Both rings armed with numerous thin, richly branched and arborescent spines (branches much thinner and more numerous than in the preceding and following species). _Dimensions._--Height of the frontal ring 0.15, breadth 0.25. _Habitat._--Equatorial Atlantic, Station 247, surface. 7. _Eucoronis challengeri_, n. sp. (Pl. 82, fig. 4). _Lithocoronis challengeri_, Haeckel, 1881, Prodromus, p. 445. Frontal ring violin-shaped, one and a half times as broad as high, with a strong sagittal constriction. Sagittal ring nearly circular, two-thirds as high as the frontal ring. Both rings armed with numerous irregularly branched spines. (The specimen figured, with red central capsule and numerous xanthellæ, was observed living by me at the Canary Islands.) _Dimensions._--Height of the frontal ring 0.1 to 0.12, breadth 0.14 to 0.17. _Habitat._--Cosmopolitan; Atlantic, Indian, Pacific, surface and at various depths. {979}Genus 419. _Plectocoronis_,[38] Haeckel, 1881, Prodromus, p. 445. _Definition._--#Coronida# with six large, partly latticed gates (four lateral and two basal). Skeleton composed of two complete rings (the sagittal and basal) and one incomplete (frontal) ring. Basal ring without developed typical feet. The genus _Plectocoronis_ differs from the preceding _Eucoronis_, its ancestral form, in the development of loose irregular lattice-work along the two meridional rings, and therefore bears to it the same relation that _Acanthodesmia_ does to _Coronidium_. If the lattice-work become more complete and close the gates, these Coronida pass over directly into #Spyroidea# (Zygospyrida). 1. _Plectocoronis anacantha_, n. sp. Basal ring and frontal ring elliptical, without sagittal constriction. Sagittal ring ovate. The frontal and sagittal rings are bordered on both edges with an incomplete series of small irregular pores. All three rings smooth, without spines. _Dimensions._--Height of the frontal ring 0.1, breadth 0.16. _Habitat._--Central Pacific, Station 274, depth 2750 fathoms. 2. _Plectocoronis triacantha_, n. sp.