Tugonia (Distugonia)

Zhang, Jun-Long, Xu, Feng-Shan & Liu, Rui-Yu, 2012, The Myidae (Mollusca, Bivalvia) from Chinese waters with description of a new species, Zootaxa 3383, pp. 39-60 : 53

Tugonia (Distugonia) sp.

Figure 7A–L

Distugonia sp. Xu, 2008: 590.

Material examined. MBM188705 (1 articulate shell), MBM188706 (1 right valve), shells collected from dead coral, intertidal zone, Sanya, Hainan Island, China by Ma Xiu-tong, on March 28th, 1958.

Description. Length to 9 mm; shell equivalve, inequilateral, elongate, thin, chalky, the anterior part of the shell more inflated than the posterior part, gaping posteriorly; posterior area small, truncated, anteriorly demarcated by weak ridge; umbo almost central, low and prosogyrate; anterodorsal margin convex, anterior margin broadly rounded; posterodorsal margin rather straight; posteroventral margin weakly curved to almost straight.

Sculpture fine, with irregular commarginal growth lines and swellings, becoming weak or disappearing in middle part, devoid of any radial rib, posterior area with strong commarginal lamellar sculpture; colour greyish with periostracum yellowish; inner shell surface with chalky blotches.

Spoon-shaped chondrophore in left valve extending posteriorly; chondrophore in right valve horizontally compressed.

Anterior muscle adductor scar thin and elongate; posterior adductor scar ovate; pallial sinus shallow, wide and barely visible; pallial line obscure.

Remarks. We think this species is new and are working to resolve this question. Tugonia (Distugonia) sp. differs from T. (D.) decurtata ( Adams, 1851) ( Figs. 6 L, 7M) by its smaller posterior area which is demarcated by a less pronounced ridge, larger and rather flat umbo, and a more strongly curved dorsal margin. Our species also differs from T. (D.) inopinata ( Iredale, 1936) ( Fig. 7 N, O; see also Lamprell & Stanisic, 1998: 7, fig. 4a–d) by its more regular shell; the rather concave posterodorsal margin of T. (D.) inopinata makes the shell more irregular; and the umbo is rather prominent.

Figures

FIGURE 7. A – L. Tugonia (Distugonia) sp., A – I, L, MBM 188705, length = 9 mm. A. Exterior of right valve. B. Exterior of left valve. Scale bar (A, B) = 2 mm. C. Hinge of right valve. Scale bar = 1 mm. D. Interior of right valve. E. Interior of left valve. Scale bar (D, E) = 2 mm. F. Hinge of left valve. Scale bar = 1 mm. G. Close up of umbo on right valve. H. Close up of umbo on left valve. Scale bar (G, H) = 1 mm. I. Close up of surface sculpture. Scale bar = 1 mm. L. Dorsal view of both valves joined. Scale bar = 2 mm. J & K, MBM 188706, length = 7.60 mm. J. Exterior of right valve. K. Interior of right valve. Scale bar (J, K) = 2 mm. M. Tugonia (Distugonia) decurtata (A. Adams, 1851) (after original figure from Sowerby II, 1875: pl. 3, fig. 11). N – O. Tugonia (Distugonia) inopinata (Iredale, 1936) (after original figure from Iredale, 1936: pl. 21, figs. 6, 7).

FIGURE 6. A – K. Tugonia (Tugonia) huanghaiensis Xu, 1987. A – F, holotype, MBM no. 73 M- 030, length = 19.8 mm. A. Exterior of right valve. B. Exterior of left valve. Scale bar (A, B) = 2 mm. C. Hinge of left valve. Scale bar = 2 mm. D. Interior of right valve. E. Interior of left valve. Scale bar (D, E) = 2 mm. F. Close up of surface sculpture. Scale bar = 2 mm. G – K, paratype, MBM no. 57 M- 101, length = 16.2 mm. G. Exterior of right valve. H. Exterior of left valve. Scale bar (G, H) = 5 mm. I. Hinge of left valve. Scale bar = 2 mm. J. Interior of right valve. K. Interior of left valve. Scale bar (J, K) = 5 mm. L. Tugonia (Distugonia) decurtata (A. Adams, 1851). From Okutani (2000: pl. 508, fig. 4, with permission from author).