Microdeuthria michaelseni Strebel 1905, new combination

Microdeuthria michaelseni (Strebel, 1905) new combination

Figures 27 A–O, 28A–G

Euthria michaelseni Strebel, 1905: 621, pl. 21, figs. 6, 6a, 6b.

? Euthria janseni Strebel, 1905: 622, pl. 21, figs. 7, 7a.

E. mulachi Strebel, 1905: 623, pl. 21, figs. 8, 8a.

Paraeuthria michaelseni (Strebel, 1905). Castellanos, 1992: 16, pl. 4, fig. 36. Paraeuthria janseni Strebel, 1905. Castellanos, 1992: 15, pl. 1, fig. 6. Pareuthria janseni (Strebel, 1905). Rosenfeld et al., 2015: 73, fig. 6D.

Type material. [Pareuthria michaelseni] 1 spm., Puerto Eugenia, Navarino Is., 10–15 fms [18.2–27.4 m] depth, clay and algae, (ZMH 3119) (ex SE6228 (768)); 1 spm., Picton Island, Banner Cove, roots of kelp, 5 m depth, ZMH 3120; Port Stanley, roots of kelp, 1 fms. [1.8 m] depth, Malvinas (Falkland) Is., ZMH 3121 (ex M196 (768)); [P. mulachi] 1 spm., Picton Is., Banner Cove, roots of kelp, 3 fms. [5.4 m] depth, ZMH 3122 (ex M168); [P. janseni], 1 spm., Picton Is., Banner Cove, roots of kelp, 3 fms. [5.4 m] depth, ZMH 3118 (M168).

Type locality. Here restricted to Puerto Eugenia, Navarino Island, Chile.

Description. Shell medium size, up to 11 mm in height, fusiform, slender, of 6 flat whorls; Shell translucent, with a characteristic white band in the middle of the last whorl, over a reddish background, some specimens with thicker shells and darker in color; protoconch (Fig. 27 N, O) subquadrate, with 2½ flat whorls, translucent, smooth, transition to teleoconch faint but visible; suture impressed; aperture elliptical, enlarged, outer lip thin; siphonal canal very short, wide; parietal callus very weak, thin; axial ornamentation absent; spiral ornamentation of very delicate lirae, 9 on the first two teleoconch whorls and about 35 on the last whorl; periostracum thin, transparent, developing distinctive wrinkles between lirae, visible only by SEM (Fig. 27 M). Operculum (Fig. 28 G) oval, subterminal nucleus.

Radula (Figs 28 A–D) rachiglossate, very small and reduced in tooth size and number; rachidian very thin, weak, with one triangular central cusp; two slightly curved, triangular lateral teeth present. Penis (Figs 28 E, F) long, cylindrical, with a large papilla on the tip rising from a hole.

Material examined. 1 shell, 34°50’S, 52°20’W, 106–124 m depth, (MACN-In 16664); 3 spms., 36°18’S, 53°52’W, 256–293 m depth, RV ATLANTIS II, Cruise 60, Sta. 280, (MACN-In 40545); 1spm., 47°43’45.6”S, 65°50’15.7”W to 47°44’8”S, 65°49’39”W, 15 m depth, (MACN-In 40548); 3 shells, 52°56’S, 75°00’W, 92–101 m depth, RV ELTANIN, Sta. 958, (USNM 887725); 2 shells, 53°51’32”S, 70°25’52”W, 2–3 m depth, (USNM 898720); 1 spm., 55°47’S, 66°17’W, 115 m depth, (USNM 870073); 2 spms., Beagle Canal, (MACN-In 40547); 1 spm., 53°59’4.3”S, 67°22’59.1”W, Estancia Viamonte, Tierra del Fuego, live collected during low tide, (MACN-In 40546); 4 spms., 54°15.043'S, 61°35.479'W, 202 m depth, CAV 2013 St. 6, (MACN-In 40549); 2 shells, 54°37.484’S, 61°9.152’W, 202 m depth, (MACN-In 40550).

Distribution. Off Buenos Aires province in deep water, Magellan Straits, Beagle Channel, Malvinas (Falkland) Is.; southern Chile and Argentina.

Remarks. Few complete specimens were available of this species. The extremely small radula with reduced rachidian teeth and simple lateral teeth serve as the basis for distinguishing a new genus. Despite the scarce material available, the thickness of the shell presented some variability. Strebel’s materials of mulachi and janseni, all from the same area, are juveniles of michaelseni and here considered synonyms.

The unique rachidian and lateral teeth with only one cusp obscure the real subfamilial affinities of M. michaelseni and, in a way, prove the taxonomic arrangement of the whole group somewhat artificial. Nevertheless, reduced lateral teeth with only a single cusp could be close to Falsitromina, which is included (as Tromina) in Cominellinae by Powell (1951). The almost obsolete central tooth of M. michaelseni shows only one cusp and in some cases a remnant microcusp on the sides which could be interpreted as tricuspid rachidian as it is usual in Cominellinae.