Thracia arienatoma, P. G. Oliver & Holmes, 2004

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3219-3221

Thracia arienatoma n. sp ( figures 152–160 )

HOLOTYPE: a live collected shell (now disarticulated valves), Gravier, Rodrigues, Stn 9A, 14.11.2000, 19 ‡ 43.9 ’ S, 63 ‡ 28.9 ’ E. NMW.Z.2000.086.00015.

PARATYPES: 1 rv as holotype, NMW . Z.2000.00016; three specimens (one as SEM mount), Pointe Coton , Stn 36, NMW . Z.2000.00017; one specimen, Stn 19, west of Port Mathurin in muddy shell sand, NMW . Z.2000.086.00018; one valve, Stn 40, NW end of Ile Gombrani , NMW .Z.2000.086.00019.

Measurements (mm, L 6 H). Holotype: 1.67 6 1.44. Paratypes: 2.30 6 1.65, 1.45 6 1.11, 1.40 6 1.13, 1.22 6 0.87, 1.56 6 1.24.

Description. Minute to 3 mm. Slightly inequivalve with lv dorsal margins fitting inside those of rv. Equilateral. Compressed. Outline subtrigonal, broadly truncated posteriorly. Beaks prominent, anterior dorsal margin straight, steeply sloping to narrowly rounded anterior margin; ventral margin straight, posterior broadly truncate, posterior dorsal margin straight, sloping less steeply than anterior dorsal marin. Posterior dorsal area developed as a flat escutcheon, anterior dorsal slightly cleft. Posterior area weakly defined by a weak ridge. Hinge feeble, rv ( figure 154 ) with well-developed anterior and weaker posterior teeth, the anterior continuous with a weak ridge appearing more strongly at the anterior end of the dorsal slope. Left valve without teeth ( figure 155 ). Ligament internal attached to a poorly developed sub-umbonal plate. Lithodesma calcified, simple ( figure 156 ). Sculpture distinctly granulose ( figure 159 ), microstructure of closely spaced wrinkled cones topped by a crystalline disc (resembling a slice of banana) which is depressed centrally ( figure 160 ). Prodissoconch ( figure 157 ) a simple dome with a well-demarcated rim, 190 M m in diameter. Muscle scars poorly defined, pallial sinus broad and very shallow.

Derivation of name. Latin ariena ‘the fruit of the plantain (banana)’ and Latin toma ‘slice’, referring to the appearance of the microsculpture.

Comparisons. Studies on Indo-Pacific Thraciidae are scant with modern works covering taxa from the north-west Pacific ( Kamenev and Nadtochy, 1998; Kamenev, 2002) and New Zealand ( Marshall, 2002). From these works the Rodrigues shells most closely resemble Thracia (Parvithracia) suteri from New Zealand but the distinctive sculpture is unlike any species included. Known taxa from the western Indian Ocean are few and include T. capensis Sowerby, 1889 , T. alfredensis ( Bartsch, 1915) and Thraciopsis agulhasensis Barnard, 1964 , but none is similar. Thracia kowiensis Turton, 1932 (NMW.Z.1955.158.815.) ( figure 161 ) was considered to represent juveniles of T. capensis ( Barnard, 1964) . We cannot confirm this but can, because of the different microsculpture ( figures 162, 163 ), state that T. kowiensis is not conspecific with T. arienatoma .

Figures

FIGS. 152–157. Thracia arienatoma n. sp., NMW.Z.2000.086.00015: (152) external left valve (MontageTM); (153) internal left valve (SEM); (154, 155) hinge views (SEM); (156) lithodesma (SEM); (157) prodissoconch (SEM).

FIGS. 158–163. (158–160) Thracia arientoma n. sp., NMW.Z.2000.086.00015 (SEM): (158) external right valve; (159, 160) sculpture. (161–163) Thracia kowiensis, NMW.Z.1955.158.815 (SEM): (161) external right valve; (162, 163) sculpture.