Scientia Sinica

162 SCIENTIA SINICA Vol. V

Ventral valve evenly and gently convex in lateral profile, the medial portion of the valve being most convex; lateral regions flatly convex; front of the valve slightly flattened; umbo less swollen; beak pointed; interarea apsacline, moderately curved.

Dorsal valve flatly convex in lateral profile; unevenly convex in the anterior half of the valve; the median region being flattened or depressed into a well marked sulcus; median sulcus moderately deep, originating from the beak of the valve and widely extended toward the anterior margin; beak small; interarea short, anacline.

Interior: Teeth strong, ponderous; dental plates low, obsolete; muscular field subcordate, adductor and diductor scars not clearly separated; a short and weak median ridge extends from the anterior of the adductors. ~

Cardinalia strong; brachiophores thick, ventrally carinate; cardinal process moderately high, expanded in front; notothyrial platform thickened, produced anteriorly as a broad median ridge about to the middle of the valve.

Measurements (mm)

Holotype, LP.A.S. 7994, paratypes, I-P.A.S. 7995a-b.

Horizon and locality: Shitzepu Formation, Middle Ordovician; north of Tungkungtze Village, Tsunyi District, Kweichow Province.

Collector: V.K. Ting.

Discussion: This species is based on two complete specimens which are coarsely silicified to retain the delicate structures of the shell. One of the complete specimens was opened in the hope of obtaining a good internal mold. However, the matrix was proved to be too coarse and porous to preserve the pallial and ovarian markings.

This species approaches Glyptorthis balclatchiensis (Davidson)™? in its size and outline but differs in having a much shallower ventral valve, a broader hinge-line, and a more incurved beak. The surface ornamentation of G. simplex differs from that of G. balclatchiensis in being somewhat angular and less uniform, and in the imbricate lamellae developed only in the anterior

of the shell.