Yoldiella ovulum, La Perna, 2004

La Perna, R., 2004, The identity of Yoldia micrometrica Seguenza, 1877 and three new deep-sea protobranchs from the Mediterranean (Bivalvia), Journal of Natural History 38, pp. 1045-1057 : 1048-1049

Yoldiella ovulum n. sp.

( figure 2 a–g)

Type material. Holotype (right valve, MZB 40660) and four paratypes (loose valves, author’s collection)

Type locality. Cruise ‘ Eocumm 95’, southern Tyrrhenian (off northern Sicily), st. 27B, 38 ° 32 ∞ 26 ◊ N, 15 ° 05 ∞ 02 ◊ E, 1096 m .

Material examined. Cruise ‘Eocumm95’, southern Tyrrhenian: type materialst. 10, 38 ° 13 ∞ 27 ◊ N, 14 ° 24 ∞ 50 ◊ E, 1357, two vs; st. 15, 38 ° 29 ∞ 06 ◊ N, 14 ° 11 ∞ 39 ◊ E, 1521 m, two vs; st. 16, 38 ° 33 ∞ 42 ◊ N, 14 ° 13 ∞ 59 ◊ E, 1340 m, one v.; Cruise ‘Interreg Italia- Albania’, southern Adriatic, st. 66, 40 ° 50 ∞ 37 ◊ N, 18 ° 28 ∞ 05 ◊ E, 759 m, one v.

Description. Shell small, almost equilateral, ovate, elongated, barely rostrated, moderately inflated, relatively solid. Umbo at midline, small and moderately projecting from shell outline, opisthogyrate. Antero-dorsal margin convex, shorter than postero-dorsal one, smoothly sloping to form a broadly rounded anterior margin. Postero-dorsal slightly convex, gently sloping to form a short, well-rounded rostrum at shell height midline. Ventral margin wide, evenly curved, moderately convex. Surface with fine growth lines, becoming well incised, irregularly spaced lines towards ventral margin. Hinge plate moderately thick, making a 130 ° angle, anterior row a little longer than posterior one. Dentition taxodont, with chevron-shaped teeth, numbering seven anteriorly and six posteriorly in holotype. Ligament pit shallow, elliptic. Muscle scars roundish, fairly well impressed. Pallial line forming a shallow, hardly distinct posterior sinus. Prodissoconch roundish, matt, ca 210 µ m long. Holotype 1.90 mm in length, 1.21 mm in height, 0.30 in breadth. Paratypes 1.90 to 1.82 mm in length.

Etymology. After the Latin word ovulum (small egg), used in apposition with reference to the markedly ovate outline.

Distribution. Known from the southern Tyrrhenian and the southern Adriatic, from about 750 to 1500 m.

Remarks. Yoldiella ovulum n. sp. is similar to Y. micrometrica , from which it differs by several shape traits all contributing to give a markedly ovate outline, rather than somewhat trigonal. Shell shape provides a good estimate for distinguishing closely similar protobranchs and the differences in outline between these two species are not negligible. Both species are similar to Y. seguenzae Bonfitto and Sabelli, 1995 , a much larger species (up to about 5 mm), which in turn is similar to Y. philippiana (Nyst, 1845) . However, juveniles of Y. seguenzae and of Y. philippiana are shorter and higher and have a smaller prodissoconch (about 180 µ m).

The species from off Israel (no depth data) reported as Yoldia micrometrica by Giannuzzi-Savelli et al. (2001: figure 61) may be Y. ovulum n. sp., but its size is markedly smaller (1.1 mm) and the hinge is unusually strong for this size.