Philobrya atlantica Dall 1896
Creators: name: Urcola, Matias Ricardo
name: Zelaya, Diego Gabriel
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Arcida
Philobryidae
Philobrya
Philobrya atlantica

description: Philobrya atlantica Dall, 1896
(Figure 4)
Philobrya atlantica Dall, 1896: 16, 17; Bernard 1897: 10‒12, 23, 24, figs. 1.3, 4.3, pl. 1, fig. 2; Dall 1902: 507, pl. 32, figs. 4, 5; Carcelles 1950: 75, pl. 4, fig. 68; Dell 1964: 166.
Type locality: off Spiring Bay, 48°37’S 65°46’W, 58 fathoms [106 m].
Material examined: Photographs of three syntypes (USNM 97057). Atlantic coast of Patagonia: Santa Cruz Province: Puerto Deseado: 47°45.42’S 65°52.60’W (MACN-In 44413: 1 spec, 1 v), 47°45.54’S 65°52.20’W (MACN-In 44414: 4 spec), 49°16’S 65°22’W, 100 m (MACN-In 44415: 1 spec).
Distribution: Atlantic coast of Patagonia, from Santa Cruz Province (Dall 1896, 1902; this study) to off Tierra del Fuego (Dell 1964), and South Georgia (Dell 1964); 100‒ 118 m.
Description: Shell small (maximum L = 4.2 mm), modioliform, relatively high (H/L = 0.81 ± 0.04, n = 6), somewhat inflated (W/H = 0.61 ± 0.05; n = 6), delicate; with a wide, ill-defined dorsal slope. Umbo broad, only slightly outstanding from shell outline, located at anterior end, somewhat dorsally displaced. Umbonal angle: 110‒120° (n = 8). Dorsal margin with the anterior portion long, straight to slightly convex, markedly obliquely 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, only slightly projected ventrally, of 470‒ 530 µm Lp (n = 6); separated from the dissoconch by a prominent rim. Central area sculptured with 10‒13 strong radial ribs, some extending from the umbo to the shell border, others are shorter, intercalated at the distal part. Antero-dorsal and postero-dorsal areas raised, with well-marked commarginal folds.
Dissoconch whitish, shiny. Sculpture weakly cancellate, consisting of 9‒12 faint radial ribs, and numerous, widely separated, narrow commarginal cords. Commarginal sculpture slightly weaker than radial sculpture.
Periostracum very thick, pale yellow, extending up to 1 mm beyond calcified shell margin; forming prominent radial folds, raised commarginal lamellae, and short, broad setae. Commarginal lamellae extending up to the tip of the setae, but forming a deep sinuation between setae.
Inner shell surface with 4‒5 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 150‒160° (n = 4). In specimens longer than 4.1 mm some distal teeth, of both anterior and posterior series, partially disintegrated, forming pustules. Resilifer long and narrow.
Remarks: The combination of the modioliform shape, radially sculptured prodissoconch, broad periostracal setae and elongate resilifer are diagnostic characters of P. atlantica.
Dall (1896) originally described the hinge of P. atlantica as “smooth”. However, the study of the syntypes of P. atlantica reveals that anterior and posterior series of G1b teeth are actually present in the type series, such as in all other specimens studied herein.
The prodissoconch of Philobrya atlantica shows the ventral margin only slightly projected, a fact resulting in an extremely low profile (Bernard, 1897; this study). This condition strikingly contrasts with that found in the other philobryid species studied herein, where the ventral margin is markedly convex, consequently producing a high profile of the prodissoconch. The prodissoconch sculpture of P. atlantica resembles that of P. alata, although the alated shell outline of the latter, allows an easy distinction of these two species.
In general shell outline, P. atlantica resembles P. wandelensis and P. sanjuani, which, however, have narrower setae, shorter resilifer and the prodissoconch sculptured with pits. In addition, P. wandelensis and P. sanjuani show the inner shell margins completely crenulated, while in P. atlantica crenulations are restricted to the posterior portion of dorsal margin (see Appendix 1). Other species with modioliform shape are P. brattstromi Soot-Ryen, 1957 (Fig. 8) and the small-sized specimens of P. blakeana (Melvill and Standen, 1914) (Fig. 6) (see comparisons under those species and Appendix 1).
description: Published as part of Urcola, Matias Ricardo & Zelaya, Diego Gabriel, 2024, The Genus Philobrya J. G. Cooper, 1867 (Bivalvia: Philobryidae) In Patagonia And Adjacent Antarctic Waters, pp. 151-192 in Zootaxa 5437 (2) on pages 158-159, DOI: 10.11646/zootaxa.5437.2.1, http://zenodo.org/record/10984619