Philobrya brattstromi Soot-Ryen, 1957

(Figure 8)

Philobrya brattstrØmi Soot-Ryen, 1957: 2 .

Philobrya brattströmi . Soot-Ryen 1959: 22, 23, pl. 1, fig. 6.

Philobrya brattstromi . Ramorino 1968: 198, pl. 1, figs. 5, 6 and pl. 5, figs. 2, 4; Malchus 2006: 8‒14, figs. 1‒23; Cárdenas et al. 2008: 227, fig. 6.70; Malchus & Sartori 2013: figs. 25.3‒5; Valentich-Scott et al. 2020: 142, pl. 47.

? Philobrya olstadi . Linse 2002: 120, fig. 9.1.2-16. (Not Soot-Ryen, 1951: 12, 13, pl. 1, figs. 9, 10).

Type locality: South of Isla Guar, W of Bajo Pucarí, Seno Reloncaví, 41°44.42’S 72°55.75’W, 70 m.

Material examined: Photographs of the holotype (NRM 3894). Beagle Channel: Isla Conejo, 54°50.88’S 68°14.67’W, 16 m (MACN-In 44430: 6 v); Isla Gable, 54°55’S 67°21’W, 15‒20 m (MLP-Ma 16113: 1 v); Isla Becasses, 54°57’S 67°01’W, 30‒40 m (MLP-Ma 16114: 2 spec, 1 v); Cabo San Pío: 55°03’S 66°37’W, 30‒35 m (MLP-Ma 16115: 1 spec, 2 v), 55°06’S 66°29’W, 65‒80 m (MLP-Ma 16116: 2 v). Atlantic coast of Patagonia: Tierra del Fuego Province: 53°10.74’S 67°13.19’W, 78 m (MACN-In 44431: 1 spec) ; 54°08.76’S 64°57.60’W, 110 m (MACN-In 44432: 1 v). Isla de los Estados: Puerto Vancouver, 54°52.44’S 64°03.37’W, 271 m (MACNIn 44433: 1 v). Burdwood Bank: 54°31.68’S 61°27.98’W, 137 m (MACN-In 44434: 1 spec, 1 v); 54°28.49’S 60°56.46’W, 102 m (MACN-In 44435: 20 v); 54°29.74’S 60°12.59’W, 95 m (MACN-In 44436: 1 v); 54°35.39’S 58°37.85’W, 140 m (MACN-In 44437: 1 v); 54°11.54’S 58°19.88’W, 90 m (MACN-In 44438: 1 spec, 9 v).

Distribution: Pacific coast of Patagonia, from Valparaíso (Ramorino 1968; Valentich-Scott et al. 2020) to Cape Horn (Linse 2002), extending into the Atlantic coast of Patagonia to Isla de los Estados and Burdwood Bank (this study); 15‒271 m (living specimens: 25‒137 m).

Description: Shell small (maximum L = 5.8 mm), modioliform, relatively high (H/L = 0.76 ± 0.03, n = 10), compressed (W/H = 0.54 ± 0.03, n = 10), delicate; with a narrow, flat to slightly depressed dorsal slope. Umbos broad, only slightly outstanding from shell outline; located at anterior end, somewhat dorsally displaced. Umbonal angle: 100‒120° (n = 8). Dorsal margin with the anterior portion long, straight, obliquely directed backwards and the posterior portion broadly curved, forming a continuous curve with the posterior margin. Ventral margin nearly straight.

Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 590‒660 µm Lp (n = 8), set off by a prominent rim. Antero-dorsal and postero-dorsal areas raised, with prominent commarginal folds near the umbo, nearly smooth distally. Central area smooth in appearance.

Dissoconch whitish, dull; sculpture cancellate, consisting of 9‒13 fine radial ribs, and numerous, moderately separated, commarginal cords. Commarginal sculpture weaker than radial sculpture.

Periostracum thick, yellow, extending up to 1 mm beyond calcified shell margin; forming narrow radial folds, raised commarginal lamellae and long, thin setae. Commarginal lamellae extending up to the tip of the setae, with straight margin between setae.

Inner shell surface with 3‒5 prominent folds radiating from the umbo to the posterior portion of dorsal margin, where produce strong crenulations. Posterior and ventral margins smooth. Posterior adductor muscle scar located in the posterior third of the valves, dorsally displaced.

Hinge: Anterior and posterior series of G1b teeth forming an angle of 167‒170° (n = 4). All teeth in both series well-recognizable in specimens of up to 2.6 mm L. Distal teeth of the posterior series partially disintegrated, forming pustules, in specimens longer than 3.3 mm. Distal teeth of the anterior series also forming pustules in specimens longer than 4.9 mm. Resilifer long and narrow.

Remarks: In general shell shape, Philobrya brattstromi resembles P. atlantica (Fig. 4), P. wandelensis (Fig. 18), P. sanjuani (Fig. 16) and small-sized specimens of P. blakeana (Fig. 6), from which it differs by having a centrally smooth prodissoconch ( P. atlantica shows radial ribs, P. blakeana radial and commarginal elements, and P. wandelensis and P. sanjuani microscopic pits). The periostracal setae of P. brattstromi are narrower than those of P. atlantica, longer than those of P. wandelensis and P. sanjuani, and sparser than those of P. blakeana (Appendix 1). In addition, the commarginal periostracal lamellae of P. brattstromi show straight margins, while those of P. atlantica, P. wandelensis, P. sanjuani and P. blakeana show a deep sinuation between setae. Philobrya brattstromi also differs from P. wandelensis and P. sanjuani by having a narrower dorsal slope of the shell, longer resilifer, and internal crenulations restricted to the posterior portion of dorsal margin (instead of having all inner margins crenulated).

Linse (2002) identified as P. olstadi a “prodissoconch” (actually a “larva”) collected in the Drake Passage, off Cape Horn. The photograph she provided (Linse 2002: fig. 9.1.2-16) is about 650 µm Lp, agreeing both in size and sculpture with the characteristics here recognized for P. brattstromi . Ríos et al. (2003) mentioned P. brattstromi from the Strait of Magellan, although this record could not be verified.

Huber (2010) regarded P. brattstromi as a junior synonym of P. aequivalvis, a criterion not shared herein.