Definition.—Theopilida (vel Tricyrtida triradiata aperta) with three latticed free lateral wings on the sides of the thorax, which are not prolonged into the cephalis.
The genus Dictyoceras and the following Pteropilium agree with the two preceding genera in the possession of three lateral wings on the sides of the thorax; but whilst these in the latter are simple solid spines, they are in the former vertical fenestrated plates. Dictyoceras may be derived from Pterocorys by development of a vertical lattice-plate between the thorax and the three free lateral spines arising from it.
1. Dictyoceras insectum, n. sp. (Pl. [71], figs. 6, 7).
Shell with two distinct strictures. Length of the three joints = 1 : 3 : 2, breadth = 2 : 4 : 3. Cephalis hemispherical, with a slender pyramidal horn of twice the length. Thorax subspherical, in the upper half with three divergent lattice-wings of the same length, each of which represents a slender, fenestrated, three-sided pyramid. Abdomen subovate, with wide, truncate mouth (in the figured specimen broken off). Pores circular, small, of different sizes.
Dimensions.—Length of the three joints, a 0.025, b 0.055, c 0.04; breadth, a 0.035, b 0.08, c 0.06.
Habitat.—Central Pacific, Stations 263 to 274, depth 2350 to 2925 fathoms.
2. Dictyoceras formica, n. sp. (Pl. [71], fig. 8).
Shell with obliterate collar, but deep lumbar stricture. Length of the three joints = 1 : 2 : 3, breadth = 2 : 5 : 4. Cephalis conical, with an oblique curved horn of the same length. Thorax with three vaulted swellings between the three prominent, diverging lattice-wings, the apex of which is prolonged downwards into a solid curved spine, half as long as the inflated abdomen. Mouth little constricted. Pores irregular, roundish, of different sizes.
Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.06; breadth, a 0.03, b 0.1, c 0.09.
Habitat.—North Atlantic, Station 353, surface.