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Lithomelissa gelasinus O´Connor, 1997

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Lithomelissa gelasinus O'Connor n. sp.
Plate 2, figures 3-6; plate 6, figures 6-9

Description: Cephalis truncate spherical, poreless but dimpled where pores would otherwise be. Surface rough due to irregularly distributed nodes and raised bars. Short, conical to weakly bladed apical horn usually present, anteriorly offset. Collar stricture weakly defined externally as a constriction and change in surface texture of shell. Thorax cylindroid. longer than, but generally same width as cephalis. Walls spongy, pores circular to subangular, irregular in size. Aperture not constricted, termination ragged. Thorax projects from distal cephalis a short way up from base of cephalis.

Dimensions: Holotype (range of 25 specimens): length of apical horn: 18pm (10-25); length of cephalis excluding apical horn: 55pm (44-56); width of cephalis: 60pm (53-70); length of thorax: 90pm (55-90); width of thorax: 70pm (55-78).

Etymology: Gelasinus is Latin for dimple and refers to the dimpled surface of the cephalis of this species. It is used as a noun in apposition.

Holotype: R153 (Plate 2, figures 3, 4).

Type Locality: Matakohe Quarry, Q08/179638.

Discussion: Lithomelissa gelasinus differs from similar looking forms in the following ways - from L. sphaerocephalis Chen (1975. p. 457. pi. 8. figs. 1, 2; Takemura 1992, pi. 4, figs. 8, 9) by having a cylindroid. rather than conical, thorax, a much shorter apical horn, by the thorax being spongy, rather than covered in a spongy network, and by being much shorter and narrower; from L. microptera, L. mitra, L. ehrenbergi, L. hertwigi (see Bütschli 1882, pi. 43, figs. 2la-22b, 24,26) and L. bütschlii (illustrated in Nishimura 1990, fig. 15-5-8) by having a poreless cephalis and spongy thorax. The structure of the internal skeleton of L. gelasinus appears similar to that of the type species, L. microptera, as illustrated in Petrushevskaya (1981, fig. 69) and Biitschli (1882, pi. 43, fig. 26). although there is little detail shown in these illustrations (Petrushevskaya's illustration is a posterior view in the sagittal plane and Biitschli's is anterior). L. sphaerocephalis also seems to have a similar internal skeletal structure, as do L. mitra. L. ehrenbergi, L. hertwigi and L. bütschlii.

Range: Lithomelissa gelasinus has been observed from the Cryptocarpium ornatum Zone to the mid-Lychnocanium elongata Zone (early to late Oligocene (late Runangan/early Whaingaroan to late Duntroonian/early Waitakian)).
O´Connor 1997











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