Halysestheria biformis Zhang & Chen

Halysestheria biformis Zhang & Chen in Zhang et al., 1976 emend.

(Figs 2–5)

1976 Halysestheria biformis Zhang & Chen in Zhang et al., p. 211, pl. 91, figs 1–2; pl. 92, figs 1–4; pl. 93, fig. 8.

1985 Halysestheria biformis Zhang & Chen (in Zhang et al., 1976), Chen & Shen, p. 133, pl. 13, figs 1–2.

Emended diagnosis. Carapace moderate in size; elliptical, long oval or subquadrate in outline; umbo located in the anterior part of the dorsal margin. Growth lines are serrated in the lower margins. Growth bands in the umbonal area ornamented with widely spaced radial lirae, intercalated with fine reticulation and puncta; growth bands in the ventral part of the carapace ornamented with chain-like reticulation, which appears as radially aligned, discontinuous, short, fine ridges on external mould.

Locality and horizon. Jilin Province, and Ganhe, OroqenAutonomous Banner, Inner Mongolia,northeastern China; First Member of the Upper Cretaceous Nenjiang Formation.

Description. Carapace moderate in size, elliptical, long oval, or subquadrate in outline. Anterior margin widely rounded, posterior margin pointed. Umbo located in the anterior part of the dorsal margin. Growth lines prominent, 20–30 or more in number, serrated along their lower margins (Figs 2E, 3B, E, G, 4A). Growth bands narrow in the umbonal area, become wider in the middle part of the carapace (Figs 2A, 5E–H); those in the ventral part of the carapace become narrower again. Growth bands near the umbo ornamented with widely spaced long radial lirae, which are simple or branched, intercalated with fine reticulation, puncta occur within the fine mesh (Figs 2B, 3C). Growth bands in the ventral part of the carapace, radial lirae are thickened, fine mesh between the lirae are radially aligned to form chain-like ornamentation (Figs 2F, 3H, 4D, F, H), which appears as radially aligned, discontinuous, short, fine ridges on external mould (Figs 2E, 3G, 4C, E, G). Growth bands in the antero-ventral part of the carapace are almost smooth with some pits in the lower part of each band (Fig. 3F).

Dimensions. In order: specimen no., number of growth bands, length (mm), height (mm), height/length ratio: NIGP27106, 26, 6.8, 4.6, 0.67; NIGP174748, 31,8.4, 5.9, 0.71; NIGP174749, 33, 7.7, 5.3, 0.7; NIGP174750, 25, 8.5, 5.9, 0.69; NIGP 174751, 21, 7.8, 5.2, 0.67; NIGP 174752,>14, 9.2, 6.8, 0.74; NIGP174753, 34, 7.5, 4.7, 0.63; NIGP174754,>21, 7.3, 4.6, 0.64; NIGP174755, 26, 7.0, 4.3, 0.61; NIGP174756, 25, 7.6, 5.1, 0.66; NIGP174757, 22, 7.7, 4.6, 0.60.

Remarks. In order to make a comparison, the holotype specimen of the type species (an external mould of a right valve) was also examined under the SEM (Fig. 2B–F). Although the specimen was varnished in order to protect it, clear micrographs have been taken from growth bands near the umbo (Fig. 2B, D) and in the ventral part of the carapace external mould (Fig. 2E). Clear widely spaced radial lirae with intercalated reticulation and puncta in the dorsal part of the carapace, and a chain-like ornamentation occurs on growth bands in the ventral part of the carapace.

The specimens from Inner Mongolia have elliptical, long oval or subquadrate outline, the height/length ratio between 0.61 and 0.74. In consideration of the sex dimorphism, the long oval form may be male, and subquadrate form may be female.

Except for the type species, several other species have also been described from the First Member of the Nenjiang Formation, such as Halysestheria ovata (Chang, 1957), H. qinggangensis Chen in Zhang et al., 1976, H. rhombica Zhang & Chen in Zhang et al., 1976, H.? sublineata Zhang & Chen in Zhang et al., 1976, H. subquadrata Zhang & Chen in Zhang et al., 1976, H. subtermina (Chang, 1957), H. yui (Chang, 1957). Li (2005) considered that H. qinggangensis and H. subtermina are junior synonyms of H. yui, the former two species have an elliptical carapace, and H. yui has a rounded carapace. The rounded carapace may be female, and the elliptical carapace may represent the male. While the H.? sublineata, having a long oval carapace, was named basing on only one specimen.It could be a junior synonym of the type species. H. subquadrata has a subquadrate carapace, which associates with H. yui, H. ovata and H. subtermina (Zhang et al., 1976), thus, may be they are the same species. H. rhombica is similar with the type species, and was named basing on only one broken specimen. It could be a junior synonym of the type species.