Cosa tholiatus, 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 : 3186-3187

publication ID	https://doi.org/ 10.1080/00222930410001695123
persistent identifier	https://treatment.plazi.org/id/03B33A71-5638-FF8A-3514-DF7215E0F936
treatment provided by	Felipe
scientific name	Cosa tholiatus
status

Treatment

Cosa tholiatus View in CoL n. sp.

( figures 44–49 View FIGS )

HOLOTYPE: 1 lv from Grand Pate´ , off Port Mathurin, shell gravel, 17 m, 119 ‡ 39.307 ’ S, 63 ‡ 24.950 ’ E. NMW.Z.2001.061.00048.

PARATYPES: 2 rv gold-coated and 10 lv and 9 rv from Grand Pate´ , off Port Mathurin, shell gravel, 18 m, NMW .Z.2001.061.00016.

Measurements (mm, L 6 H). Holotype: 1.35 6 1.60. Paratypes gold-coated: 1.30 6 1.60, 1.25 6 1.62; others: 1.76 6 1.98, 1.29 6 1.58, 0.87 6 1.03, 1.12 6 1.35, 0.94 6 1.08, 1.11 6 1.36, 1.22 6 1.30, 0.95 6 1.05, 1.44 6 1.71, 1.71 6 2.11, 1.36 6 1.70, 1.01 6 1.02, 1.36 6 1.61, 1.42 6 1.62, 1.44 6 1.77, 1.23 6 1.44, 1.43 6 1.53.

Description. Size to 1.6 mm. Equivalve. Inequilateral; umbo situated anteriorly, one-quarter distance from the anterior end of the hinge plate. Outline obliquely trapezoidal; dorsal margin long, straight, sloping slightly to the posterior; dorsal anterior margin very short, straight, forming a shallow byssal notch at junction with rounded anterior margin; ventral margin rounded; posterior margin long, gently curved to almost straight. Hinge without prionodont teeth but retaining two sets of provincular denticles ( figure 47 View FIGS ) either side of the ligament pit; anterior set numbering about 12, posterior set about 24 in number increasing in size towards the margins. Ligament pit small, triangular, shallow, not extended posteriorly (aclinal). Sculpture sub-cancellate with radial elements more strongly developed than concentric elements especially on posterior and anterior surfaces. Overall, with about 30 radial riblets with underlying, weak concentric lines creating weak nodules at their intersections. Prodissoconch (PdI) about 270 M m ( figures 48, 49 View FIGS ), prominent, hat-shaped with an elevated median boss. Outer surface of brim vertical, top flat but irregularly crimped with raised radial lines running towards the boss and also with a few concentric ridges close to the dorsal edge. Inner margin smooth. Shell translucent, myostracum opaque.

Derivation of name. Greek tholia ‘a comical broad brimmed hat’, referring to the shape of the prodissoconch.

Comparisons. Cosa waikikia ( Dall, Bartsch and Rehder, 1938) has a goldenyellow shell with distinct radial sculpture and a few posterior ventral, inner margin crenulations. These characters also apply to the Japanese cave material illustrated by Hayami and Kase (1993) under the same name. These characters are absent from the Rodrigues material.

Hayami and Kase (1993) describe a second species as Cosa sp. indet., which like the Rodrigues material lacks inner marginal crenulations and has a weak sculpture. However, the prodissoconch sits at the anterior extremity of the dorsal margin, and the rim is crenulated rather than having the vertical-sided outer edge of the Rodrigues material.