Indosphenia sowerbyi (EA Smith, 1893)

Oliver, P. Graham, Hallan, Anders, Jayachandran, P. R., Joseph, Philomina, V. F. Sanu, & Nandan, S. Bijoy, 2018, Taxonomy of myid bivalves from fragmented brackish-water habitats in India, with a description of a new genus Indosphenia (Myidae, Myoidea, Myidae), ZooKeys 799, pp. 21-46 : 35-37

Indosphenia sowerbyi (EA Smith, 1893)

Sphenia sowerbyi EA Smith, 1893: 280, pl. 15, fig. 8.

Type material examined.

Syntypes, 5 shells, Ariancoupar near Pondicherry [this can be interpreted as the Chunnambar River mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E]. NHMUK 1893.3.16.6-10.

Other material examined.

4 shells, NMW 1955.158 15753 and 50+ shells, NHMUK 1953.1.7, all from Adyar, Madras [this can be interpreted as the Adyar River Mouth, Adyar, S of Chennai], 1 3°0.8'N, 80°15.4'E, all Winckworth Collection, March 1931 to June 1936.

50+ shells, Cooum River, Madras [this can be interpreted as the Cooum River mouth, Chennai, 13°4.3'N, 80°16.1'E]. Winckworth Collection, September 1931.

Type locality.

Ariancoupan, Pondicherry [this can be interpreted as the Chunnambar River Mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E].

Description.

The sample from Adyar most closely matches the form of the syntypes (Fig. 8 l–o) from Ariyankuppam, and is described here as typical. Shell (Fig. 8 a–e) to over 12 mm in length, inequivalve, left valve slightly smaller; slightly inequilateral, beaks slightly to the posterior (PL/AL = 0.83-0.97); moderately inflated (L/T = 1.9- 2.2). Beaks prosogyrate, directed anteriorly. Outline subovate-sub-rostrate, anterior end broadly rounded, ventral margin curved, L/H = 1.6-1.8; posterior end much narrower than anterior end, sub-rostrate, demarcated initially by sharp carina running from beak to ventral margin of posterior end, but this carina rapidly becoming obsolete; posterior dorsal margin concave; posterior end rounded, often eroded and obscured by periostracum extending beyond shell. Sculpture of weak commarginal lines and dense, thin raised lamellae - these more prominent on angle of rostrum. Exterior of shell with persistent, thin, straw-coloured periostracum. Prodissoconch (Fig. 9c) small P1 (75 µm) much larger P2 (189 µm); P1 with punctate micro sculpture, P2 with commarginal ridges. Hinge myid, with chondrophore in left valve. Chondrophore (Fig. 7e) with proportionately wide ligament insertion plate and short posterior flange with rounded margin; ridge between ligament area and flange prominent. Right valve with very weak pseudo cardinal tooth (Fig. 7i) commonly rising dorsally to obscure beak. Muscle scars poorly defined. Anterior adductor muscle scar elongate and placed medially on anterior face. Posterior adductor muscle scar subcircular, placed close to dorsal margin. Pallial sinus a shallow depression, more sinusoidal than U-shaped (Fig. 8d).

Intraspecific variation.

Comparisons of shells from the Adyar and Cooum rivers show some significant difference in shape, despite the rivers being only 6 km apart. The shells from the Cooum River (Fig. 8 g–i, k) are shorter posteriorly, thus the beaks are closer to the posterior end, and the height and tumidity are proportionately greater. The shells from the Cooum River are more robust and about 20% are tinged with pale purple (Fig. 8g). Although the tissues are dry, the blotched pattern (Fig. 8j) on the mantle has been preserved and it matches that present in both I. abbreviata and I. kayalum .

Remarks. Indosphenia sowerbyi

has a thicker shell than either I. abbreviata or I. kayalum and possesses a much more prominent sculpture. The beaks lie closer to the posterior end, which is the opposite of both I. abbreviata and I. kayalum . Furthermore, the posterior carina is not developed, whereas in I. abbreviata it is finely developed. Additionally, prodissoconch 2 shows no radial sculpture, as is present in I. abbreviata , making it more similar to I. kayalum in this character.