(Figures 6–7)
Mulinia byronensis Gray, 1837: 376, fig. 33; Hanley, 1843: 35, pl. 10, fig. 38; Adams & Adams, 1856: 380; Dall, 1909: 274, 286–287; Carcelles & Williamson, 1951: 343.
Mactra byronensis d´Orbigny, 1846: 511; Reeve, 1854: pl. 15, fig. 77; Weinkauff, 1884: 101, pl. 34, figs. 3–4.
Mulinea byronensis Gray. Conrad, 1868: 31.
Mactra byroni Weinkauff, 1884: 101 (nomen nudum).
Mactra byroneana Weinkauff, 1884: 101 (nomen nudum).
Mactra jousseaumi Mabille & Rochebrune, 1889: 106.
Mactra pencana Philippi, 1893: 8, pl. 1, fig. 2.
Mactra lotensis Philippi, 1893: 10, pl. 3, fig. 8.
Diagnosis. Shell oval, elongated, dorsal margin delimited by a line from umbo to posterior end; short lateral teeth close to cardinals.
Description. Shell oval to trigonal, equilateral, compressed; dorsal margins curved; line from umbo to posterior end very clear; ventral margin curved; external surface smooth with irregular growth lines, brownish periostracum (Fig. 6 A–H). Umbos prosogyrous, not inflated. Internally white with V-shaped pallial sinus of 1 / 3 of shell length; internal ligament in trigonal chondrophore ventrally developed. Right hinge with two anterior and two posterior lateral teeth, strong, each with one rounded cusp (Fig. 7 A); two cardinal teeth on anterior side of chondrophore, posterior cardinal vertically oriented; Left valve with one anterior and one posterior lateral teeth (Fig. 7 B), anterior cardinal tooth oblique in relation to dorsal margin. Ultrastructure of two layers, internal with cross lamellar complex and external with simple cross lamellar structure (Fig. 7 C–D).
Type material. [M. byronensis] NHM 196324, 5 syntypes (figs. 6 A–H); [M. pencana] MNHNC (unnumbered), holotype from Talcahuano, Chile (figs. 6 I –K); [M. jousseaumi] MNHN (unnumbered), one syntype from Orange Bay (figs. 6 L–M); [Mactra lotensis] MNHNC (unnumbered), holotype from Lota, Chile (figs. 6 N–O).
Type locality. “South America ”.
Other material examined. Peru. Peru (NHM 1563, 2 complete valves). Chile. Valparaíso, Chile (MLP 5355, 3 valves); Concepción, Chile (NHM s/n°, 4 valves); Talcahuano, Chile (MACN 13804, coll. in 1923, 26 valves); Puerto El Hambre (AMNH 34041, 1 valve); Magellanic Strait (MLP 1848, 1 valve; MZUSP 13060, 3 valves; AMNH 34039, 1 valve; NHM 79.10.15.68, coll. by Coppinger on H.M.S. Alert). Argentina. Ushuaia, Tierra del Fuego (MZUSP 22298, 2 valves, 1981).
Distribution. Peru to Ushuaia, Argentina.
Remarks. Weinkauff (1884) mentioned that Gray should have named this species Mulinia byroni or byroneana. However, these nominal taxa must be considered unjustified emendations. Mulinia byronensis is characterized by a more elongated shell than M. edulis. The posterodorsal line that delimits the posterior area in M. byronensis is absent in M. typica. Mactra jousseaumi, collected in Orange Harbor (Magellanic Strait) by the “Mission Scientifique du Cap Horn” (Fig 6 L–M), and Mactra pencana (Fig. 6 I–K) and Mactra lotensis (Fig. 6 N–O), both described by Philippi (1893) living on the Chilean coast, are clearly synonyms.