Page:Scientific results HMS Challenger vol 18 part 2.djvu/277

Rh the horn and the feet, bearing more numerous lateral branches (twelve to sixteen pairs on each foot). Columella with six to eight triradiate verticils. Horn about as long as the shell.

Dimensions.—Shell 0.1 long, 0.1 broad.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

5. Pteroscenium macropodium, n. sp.

Shell campanulate, spinulate, with irregular polygonal pores; very similar to the two preceding species, but differing in the size and shape of the three feet, which are for the most part solid, triangular, nearly parallel, vertical, very slender, three times as long as the shell, latticed only at the curved base, with few pairs of lateral branches. Columella with four to six triradiate verticils, as long as the stout triangular pyramidal horn.

Dimensions.—Shell 0.1 long, 0.08 broad.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

6. Pteroscenium tripocolpum, n. sp.

Shell campanulate, spinulate, similar to Euscenium tricolpium (Pl. 53, fig. 12) with three vaulted hemispherical bosoms between the three delicate arachnoidal vertical wings. Network loose, very irregular, with polygonal meshes of very different sizes. Columella with three or four triradiate verticils, as long as the slender pyramidal horn. Three feet twice as long, somewhat curved, widely divergent, with three or four pairs of lateral branches. The three edges of the feet and of the horn are elegantly denticulated.

Dimensions.—Shell 0.06 long, 0.09 broad.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

Definition.— (vel Monocyrtida triradiata clausa) without internal columella, with an apical horn.

The genus Peridium (or Archiperidium, Prodromus, loc. cit.) and the two following nearly allied genera form together the small group of Archibursida, differing from the four preceding genera (the Euscenida) in the absence of an internal free columella. The three basal feet have therefore no direct connection with the apical horn. Peridium, the ancestral form of the Archibursida, may have originated either from Euscenium by reduction and loss of the columella, or directly from Tripospyris by loss of the sagittal ring and the longitudinal constriction.

Definition.—Feet smooth, simple, neither spiny nor branched.