Mactra fuegiensis Smith 1905

Mactra fuegiensis Smith, 1905

(Figures 2–4)

Mactra fuegiensis Smith, 1905: 337, fig. 6; Carcelles, 1950: 81, pl. 5 fig. 90; Carcelles & Williamson, 1951: 343. Mactra epidermia Philippi, 1893: 9, pl. 1, fig. 3 (non Deshayes in Reeve, 1854).

Diagnosis. Shell trigonal, ventral margin rounded to straight, posterior area delimited by a line; periostracum brownish, almost absent in the umbonal area.

Description. Shell trigonal, inequilateral, strong, large (up to 65 mm); external surface smooth with commarginal lines close to the umbonal area; periostracum brownish and thin; anterior dorsal margin straight, posterior rounded; right hinge with two divergent cardinals (3a & 3b), unfused; anterior lateral teeth (AI & AIII), short and close to cardinals; posterior lateral teeth present (PI & PIII) (Fig. 3 B). V-shaped cardinal tooth in left valve (2a & 2b). One anterior and one posterior lateral tooth (AII & PII) running parallel to the dorsal shell margin; each elongated, fragile, with one cusp (Fig. 3 A); external ligament rudimentary, posterior to umbos; internal ligament (resilium) large, strong, in chondrophore. Muscle scar semicircular, vertically elongated, with dorsal and ventral ends pointed, close to dorsal edge; pallial sinus rounded, 1/3 of total shell length (Fig. 2D).

Ultrastructure of the shell with two layers, outer crossed lamellar of elongated crystallites arranged into lamellae; crystallites in the adjacent lamellae differ in alignment by approximately 90–98°. Inner complex crossed lamellar, consisting of intergrowth pattern resulting from crossed lamellar structure in two alignments (Fig. 3 C–D). Mantle cavity organs: Labial palps very elongated in relation to body size (Fig. 4 A). External demibranch fused to mantle. Siphons completely fused, covered by periostracum (Fig. 4 B).

Type material. [M. fuegiensis] NHM 1905.8.11.7-10, four syntypes. The dimensions of the specimen illustrated by E. A. Smith are: length 51, 5 mm, height 37, 5 mm, width 21 mm, which is clearly one of the figured specimens (Figs. 2A–D). [M. epidermia] MNHNC (unnumbered) holotype (figs. 2I –L).

Type locality. San Sebastian beach, Tierra del Fuego, Argentina. Collected by Captain Crawshay in October 1904.

Other material examined. Argentina. Puerto Madryn (ANSP 170454); San Julián bay (MLP 1850, 1857); Cape Buen Tiempo (ANSP 70091); Santa Cruz (MZUSP 13045); Magellanic Strait (MNRJ 5668); Puerto San Sebastián: (MACN 12609; USNM 898712, 20 specimens, coll. 29/7/ 1969, 18 m); Río Grande (MACN 12611); Tierra del Fuego (MORG 2742; MACN 12608; MLP 9383); Ushuaia (FURG 46331). Chile. Cape Horn, 42 m. (AMNH 271653, Vema Expedition, 1961); Punta Arenas (ANSP 103356).

Distribution. Golfo Nuevo in Argentina to Punta Arenas in Chile.

Remarks. Mactra fuegiensis was collected from Isla Grande de Tierra del Fuego coast. It is well represented in the Magellanic Province and constitutes the southernmost species of Mactra from South America. Mactra epidermia Philippi, 1893 (non Deshayes in Reeve, 1854), is a junior homonym (Fig. 2 I–L) (Mactra epidermia Deshayes in Reeve, 1854, with Portugal as type locality, has priority over Philippi’s species). Bernard (1983) wrongly included both nominal taxa as synonyms of Mulinia edulis King & Broderip, 1832. Mactra fuegiensis is characterized by a trigonal shell with a straight ventral margin and an external ligament clearly not observed in Mulinia edulis King & Broderip, 1832, which it otherwise closely resembles.