Yoldiella micrometrica (Seguenza, 1877)

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 : 1047-1048

Yoldiella micrometrica (Seguenza, 1877)

( figure 1 a–d)

Yoldia micrometrica sp. n., Seguenza, 1877a: 96.

Yoldia micrometrica, Seguenza, 1877b: 21 , pl. 4, figure 22–22c.

Type material. The type material is lost and a neotype is here designated (left valve, MZB 40659) .

Type locality. Vallone Catrica (southern Calabria), Pleistocene bathyal muddy beds .

Material examined. Four vs from the neotype locality.

Description. Shell small, slightly inequilateral, ovate-trigonal, elongated, shortly and bluntly rostrated, moderately inflated, rather sturdy for size. Umbo at midline, small and moderately projecting from shell outline, slightly opisthogyrate. Anterodorsal margin slightly convex, rapidly and smoothly sloping to form a broadly rounded anterior margin. Postero-dorsal margin straight, sloping to form a short, well-rounded rostrum, slightly below shell height midline. Ventral margin wide, evenly curved to faintly straight near midline. Surface with fine growth lines, becoming well incised, and 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 neotype. Ligament pit elliptic, shallow. Muscle scars roundish, well impressed. Pallial line fairly well impressed, forming a shallow, hardly distinct posterior sinus. Prodissoconch D-shaped, ca 300 µ m long. Neotype 2.08 mm in length, 1.31 mm in height, 0.50 mm in breadth.

Distribution. Only known from Pleistocene bathyal beds in southern Italy (Messina Strait area). The species was originally reported from a locality near Messina, where the type horizon is no longer exposed. The neotype is from a nearby locality on the Calabrian side of the Messina Strait, where beds of the same age and bathyal environment outcrop ( Di Geronimo and La Perna, 1997).

Remarks. Yoldiella micrometrica has a somewhat ‘triangular’ outline, because of the rather low position of the rostrum and the weak convexity of the ventral margin. This shape was described as ‘ trigona ’ (triangular) by Seguenza (1877a) in the brief original description. Later, when it was more extensively described, Seguenza (1877b) wrote: ‘ oblonga-subtrigona ’, ‘This very small species is well distinct by the trigonal shape…’. A well-distinct small living species has been up to now misidentified as Y. micrometrica (see below) and a neotype is here designated in order to fix the identity of the fossil species.