Philobrya multistriata Lamy 1908

Philobrya multistriata Lamy, 1908 (Figure 13) Philobrya multistriata Lamy, 1908: 128, 129, figs. 1, 2; Dell 1964: 166, text figs. 2.1, 2.10, pl. 4, fig. 6. Type locality: Îles Malouines ou Falkland [= Malvinas/Falkland Islands, 50°44’S 65°04’W]. Material examined: Photographs of one syntype (MNHN-IM-2000-34158). Atlantic coast of Patagonia: Tierra del Fuego Province: 54°08.76’S 64°57.60’W, 110 m (MLP-Ma 7459: 7 spec). Isla de los Estados: Puerto Vancouver, 54°52.44’S 64°03.37’W, 271 m (MACN-In 44466: 3 v). Burdwood Bank: 54°28.49’S 60°56.46’W, 102 m (MACN-In 44467: 1 v); 54°23.82’S 58°30.53’W, 135 m (MACN-In 44468: 1 spec, 22 v); 54°29.64’S 56°08.13’W, 290 m (MLP-Ma 7460: 5 spec). Distribution: Atlantic coast of Patagonia at Tierra del Fuego Province (Dell 1964; this study), Isla de los Estados (this study), Burdwood Bank (Dell 1964; this study) and Malvinas / Falkland Islands (Lamy 1908); 110‒ 463 m. Description: Shell medium-sized (maximum L = 7.4 mm), ovate, high (H/L = 0.96 ± 0.02, n = 11), compressed (W/H = 0.57 ± 0.06, n = 11), moderately solid; with a wide, somewhat depressed dorsal slope. Umbo broad, located at anterior end, poorly discernible in smaller specimens, somewhat projected in larger specimens. Umbonal angle: 90‒100° (n = 10). Dorsal margin with the anterior portion relatively long, nearly vertical, straight, and the posterior portion curved to flattened, forming a continuous curve with the posterior margin. Ventral margin oblique, straight to slightly arched. Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 850‒950 µm Lp (n = 10), separated from the dissoconch by a prominent rim. Antero-dorsal and postero-dorsal areas raised, sculptured with low commarginal folds. Central area sculptured with 9 ‒13 thick radial ribs. Some ribs extending all along; others (shorter), appears distally intercalated to the previous ones. Dissoconch whitish, dull. Sculpture cancellate, consisting of 10‒15 narrow, widely separated radial ribs, and numerous, moderately separated, regularly distributed, thin, commarginal cords. Commarginal sculpture weaker than radial sculpture. Periostracum thick, pale yellow, extending up to 1 mm beyond calcified shell margin; forming narrow radial folds, raised commarginal lamellae, and short, thin setae. Commarginal lamellae extending up to the tip of the setae, with straight margin between contiguous setae. Inner shell surface with 37‒48 folds radiating from the umbo to shell margins, those reaching the posterior part of dorsal margin stronger than the others. Posterior adductor muscle scar located in the antero-posterior half of the valves, dorsally displaced. Hinge: Anterior and posterior series of G1b teeth forming an angle of 161‒169° (n = 5). Distal teeth of the posterior series partially disintegrated, forming pustules, in specimens longer than 2.9 mm. Distal teeth of the anterior series also forming pustules in specimens longer than 3.2 mm. Resilifer short and wide. Remarks: The ovate shell outline and extremely large prodissoconch are distinctive characters of P. multistriata. In general shell shape, P. multistriata resembles P. limoides (Fig. 11), from which it differs by having larger prodissoconch (850‒950 µm vs. 670‒750 µm), sculptured with fewer radial elements (9‒13 vs. 42‒47). In addition, in P. multistriata the inner shell surface is sculptured with radial folds, while these elements are absent in P. limoides (Appendix 1). Furthermore, both species are allopatric in distribution: P. multistriata occurs in the Patagonian (sub-Antarctic) waters and P. limoides in the Antarctic waters (south of the Polar Front). Small-sized specimens of P. multistriata also resemble P. magellanica. However, the prodissoconch sculpture (consisting of radial ribs in P. multistriata vs. pits in P. magellanica), the sculpture of the dissoconch (cancellate in P. multistriata vs. only showing radial elements in P. magellanica), and the presence (in P. multistriata) vs. absence (in P. magellanica) of internal folds and marginal crenulations, clearly allow the distinction of these two species. Another similar species of P. multistriata is P.sublaevis (Fig. 17; see comparison under that species and Appendix 1). The personal finding of P. multistriata in Burdwood Bank and the currently known distribution of P. sublaevis (restricted to Antarctic waters) suggest that the Burdwood Bank specimens identified by Melvill & Standen (1912) as Philobrya sublaevis could actually correspond to P. multistriata. Philobrya multistriata is sympatric with P. alata, with which it shares the presence of a radially sculptured prodissoconch, cancellate dissoconch sculpture, and short resilifer. However, the evenly ovate shell outline of P. multistriata, with straight to concave ventral margin clearly allows the distinction of this species from P. alata, where the ventral margin forms a wide, convex curve with the posterior margin. Furthermore, P. multistriata shows stronger G1b teeth than P. alata.

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