Habitat.—Fossil in Tertiary rocks of the Mediterranean (Ægina, Greece; Caltanisetta, Sicily).
Genus 596. Sethornithium,[[213]] Haeckel, 1881, Prodromus, p. 436.
Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three latticed lateral wings on the thorax.
The genus Sethornithium differs from the preceding Lithornithium, its ancestral form, only in the fenestration of the three thoracic wings, and bears therefore to it the same relation that Dictyoceras exhibits to Pterocorys.
1. Sethornithium dictyopterum, n. sp. Pl. [68], fig. 14.
Shell ovate, with two slight strictures. Length of the three joints = 2 : 4 : 3, breadth = 1 : 5 : 4. Cephalis ovate, with a pyramidal horn of twice the length. From the middle part of the thorax arise three broad, triangular, latticed wings of about the same length, the distal end of each of which is curved downwards. Abdomen inversely conical. Pores regular, circular. (Similar to Lithornithium fringilla, Pl. [67], fig. 2, but with much larger wings, which in the greater part are fenestrated.)
Dimensions.—Length of the three joints, a 0.04, b 0.08, c 0.06; breadth, a 0.02, b 0.1, c 0.08.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
Genus 597. Theopera,[[214]] Haeckel, 1881, Prodromus, p. 436.
Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three lateral wings, beginning from the sides of the thorax and prolonged into the sides of the abdomen.