Retusa sp. 2

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 740-742

Retusa sp. 2

Figs 77E, F, 78

MATERIAL EXAMINED. — Coral Sea. MUSORSTOM 5: stn 322, 21°19’S, 158°00’E, 975 m, 1 dd.

DISTRIBUTION. — Single specimen examined collected from the Coral Sea (Fig. 78), in 975 m.

DESCRIPTION. — Shell morphology. Length 2.5 mm, width 1.5 mm (single specimen examined). Shell fragile, short and truncate apically, with convex sides (Fig. 77E). Only 1 whorl visible, forming nearly the entire shell, except for a small visible portion of the spire. Apex deeply umbilicate (Fig. 77F), with the aperture lip conspicuously rising from the right side. The aperture lip forms a triangular wing connected to the columellar margin. Anterior end of

FIG. 77. Shell morphology of species of Retusa Brown,1827 . A, Retusa sp. 3 , (3 mm), ventral view, Fiji, MUSORSTOM 10 CP 1341; B, same shell, posterior view. C, Retusa sp. , 4 (4 mm), ventral view, New Caledonia, BIOGEOCAL KG 278; D, same shell, posterior view. E, Retusa sp. 2 , (2.5 mm), ventral view, Coral Sea, MUSORSTOM 5 stn 322; F, same shell, posterior view.

the shell rounded. Aperture as long as the shell, wider anteriorly and narrowing gradually at about 1/2 of its length. Columellar margin slightly thickened. Columella simple, with no folds. Umbilicus closed. Sculpture of a number inconspicuous growth lines. Colour uniformly whitish.

Anatomy. Unknown. The single shell collected lacked soft parts.

REMARKS. — The shell morphology of Retusa sp. 2 resembles that of Diniatys tongana n. sp. (described below), but the former is shorter and lacks the characteristic tooth on the columella and the distinct spiral grooves near the anterior end of the shell. Another species of Retusa with a short, truncate shell is Retusa complanata Watson, 1883 , originally described from Papua New Guinea in 51 m. The holotype of this species (BMNH 1887.2.9.2198) has strong sculpture of spiral grooves and growth lines that are absent in Retusa sp. 2 .

Retusa sp. 2 appears to be distinct from other Indo-Pacific species of the genus, but it is not named here because of the lack of enough material for study. The small size of the single shell here examined suggests that this could be just a juvenile form of another species of Retusidae . This species is placed in Retusa , pending availability of complete specimens.