Scientia Sinica

164 SCIENTIA SINICA Vol. V

lateral angles rounded; lateral profile equally biconvex. Surface multicostellate, costellae being curved and increasing by repeated bifurcation. At the front margin of the shell there are about 10 costellae in 5 millimetres.

Ventral exterior: Strongly and evenly convex in lateral profile, with the greatest convexity a little posterior to the middle; umbo swollen, separated from the flanks of the shell by a concave area just in front of the posterior margin; beak prominent, incurved; interarea short, apsacline.

Dorsal exterior: Gently and evenly convex, the greatest convexity being around the middle; umbo flatly convex, slightly swollen; beak not prominent; interarea almost in the same height as the ventral one, orthocline.

Ventral interior: Teeth stout, triangular; dental plates low, receding; umbonal cavities indistinct; muscular area large, trilobate; adductor track broad and prominent, highly elevated by callus at the front; diductor tracks triangular, divergent, much shorter than the adductors; adjustor scars small, located at the base of the dental plates. Pallial markings conspicuously developed. .

Dorsal interior: Cardinalia confined; brachiophores short and blunt, supporting plates largely obscured by callus, converging toward each other to form a cruralium; sockets shallow; cardinal process a thin linear ridge; adductor scars subquadrate, deeply impressed, the anterior pair the larger; median ridge strong, extending not beyond the anterior margin of the muscle field. Pallial markings strongly developed.

Measurements (mm)

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| Length | Width | Thickness | Hinge-width Complete specimen 7999 20.5 | 23.8 12.4 16.5 Ventral valve 8000 20.3 23.8 5:3 15.2 Dorsal valve 8001 22.2 25.0 5.6 18.1

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Holotype, I-P.A.S. 7999, paratypes, I.P.A.S. 8000, 8001, 8002.

Horizon and localities: Neichia Series, Middle Ordovician; Pankuchai, west of Tangtishuya Village, north Ichang District, Hupeh Province; half a mile east of Yingwuhsi, Ssunan District, Kweichow Province.

Collectors: T. Y. Hsu and the author.

Discussion: This species is represented in our collection by abundant fine specimens. The Kweichow materials include several excellent internal molds from which all the details of the pallial markings could be determined.

The distribution of Mimedla is chiefly in the Appalachian region of North