Genus 276. Larnacoma,[1] n. gen.
Definition.—Larnacida with double lentelliptical cortical shell, without radial spines. Medullary shell double, Larnacilla-shaped.
The genus Larnacoma has originated from the nearly allied Larnacalpis by duplication of the cortical shell. Whilst in both genera the connection between the Larnacilla-shaped medullary shell and the primary cortical shell is the same, many short radial beams arise from the surface of the latter in Larnacoma, which at constant equal distances from it unite by a network forming the secondary or outer cortical shell. It differs from the similar Druppulida (Cromyodruppa) by the sagittal flattening of the lentelliptical shell and the Larnacilla-form of the double medullary shell.
1. Larnacoma lentellipticum, n. sp.
Shell with smooth surface and elliptical perimeter, one and a third times as long as broad. All four shells lentelliptical. Distance between the two cortical shells about twice as great as the distance of the inner cortical and outer medullary shell. Network of both outer shells irregular, with large roundish pores, twice to six times as broad as the bars.
Dimensions.—Length (or principal axis) of the first (innermost) shell 0.03, second 0.08, third 0.16, fourth (outermost) 0.27, breadth (or transverse axis) corresponding—(A) 0.02, (B) 0.05, (C) 0.11, (D) 0.2.
Habitat.—South Atlantic, Station 323, depth 1900 fathoms.
2. Larnacoma quadruplex, n. sp.
Shell with thorny surface and elliptical perimeter, one and a fifth times as long as broad. All four shells lentelliptical. Distance between the two cortical shells somewhat smaller than the distance between the inner cortical and outer medullary shell. Network of both outer shells irregular, with large roundish pores, twice to ten times as broad as the bars.
Dimensions.—Length of the first shell 0.02, second 0.06, third 0.15, fourth 0.24; breadth corresponding—(A) 0.016, (B) 0.04, (C) 0.11, (D) 0.2.
Habitat.—South Atlantic, Station 335, depth 1425 fathoms.
3. Larnacoma hexagonium, n. sp.
Shell with thorny surface and hexagonal perimeter, as long as broad. All four shells hexagonal, connected by six piercing radial beams (two in the principal axis, four others in two crossed diagonals). Distance between the two cortical shells somewhat greater than the distance between the inner cortical and outer medullary shell. Network of both outer shells subregular, with small circular pores, twice as broad as the bars.
- ↑ Larnacoma = Shell of chest form; λάρναξ.
