Philobrya atlantica Dall, 1896

Urcola, Matias Ricardo & Zelaya, Diego Gabriel, 2024, The Genus Philobrya J. G. Cooper, 1867 (Bivalvia: Philobryidae) In Patagonia And Adjacent Antarctic Waters, Zootaxa 5437 (2), pp. 151-192 : 158-159

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).

Figures

FIGURE 4. Philobrya atlantica. A: syntype (USNM 97057), B‒K: specimens from Santa Cruz Province (B‒F: MACN-In 44414, G‒K: MACN-In 44415). A‒E: outer views right valve (A‒D: with periostracum, E: without periostracum), F: outer view left valve (without periostracum), G: inner view left valve, H: inner view right valve, I: brooded embryo removed from the demibranch of an adult specimen, J, K: hinge plate of a 4.2 mm L specimen (J: left valve, K: right valve). Scale bars A‒H: 1 mm; I: 100 µm; J‒K: 500 µm.

FIGURE 6. Philobrya blakeana. A, I: lectotype of Brachidontes (Hormomya) blakeanus (MMUM EE 7674), B, J‒L, O, P: specimens from Tierra del Fuego (B: MACN-In 44420, J, K, O, P: MLP-Ma 16106, L: MLP-Ma 16108), C‒H, M, N: specimens from Burdwood Bank (MACN-In 44422). A‒C, E‒H: outer views right valve (A, B: with periostracum, C, E‒H: without periostracum), D: outer view left valve (without periostracum), I, J: inner views right valve, K: inner view left valve, L: brooded embryo removed from the demibranch of an adult specimen, M‒O: hinge plates right valve (M: specimen of 7.7 mm L, N: specimen of 5.2 mm L, O: specimen of 3.5 mm L), P: hinge plate left valve (specimen of 3.5 mm L). Scale bars A‒K: 1 mm; L: 100 µm; M‒P: 500 µm.

FIGURE 8. Philobrya brattstromi. A‒C, H, I, L‒N: specimens from Burdwood Bank (A, B, L: MACN-In 44438, C, H, I, M, N: MACN-In 44435), D, F, G, J, K: specimens from Tierra del Fuego (D: MACN-In 44432, F, G: MACN-In 44431, J, K: MLP-Ma 16114), E: holotype (NRM 3894). A‒D, F, J: outer views right valve (A: with periostracum, B‒D, F, J: without periostracum), E, G: outer views left valve (E: with periostracum, G: without periostracum), H: inner view left valve, I: inner view right valve, K: detail of prodissoconch, L, M: hinge plates left valve (L: specimen of 4.9 mm L, M: specimen of 3.3 mm L), N: hinge plate right valve (specimen of 3.2 mm L). Scale bars A‒I: 1 mm; J: 200 µm; K: 100 µm; L‒N: 500 µm.