Scientia Sinica

168 SCIENTIA SINICA Vol. V

1922, P. borealis, Hayasaka: Sci. Rep. Tohoku Imp. Univ., Vol. V1, No. i, pp. 28-29, pl. 1, figs. 9a-c.

1925, P. borealis, Grabau: Bull. Geol. Surv. China, No. 7, p. 80.

Shell very large; elongate-subovate in outline, anteriorly widened; lateral profile plano-convex; lateral commissures almost straight; anterior commissure broadly unisulcate; hinge-line relatively short, commonly equal to only onethird the maximum width of the shell.

Ventral valve very strongly convex in lateral profile, highest region a little posterior to middle of the valve, whence sloping first smoothly and then rapidly toward the anterior margin; shell posterior to the umbo bending abruptly dorsoward; anterior profile rather evenly and broadly convex; lateral slopes bending abruptly downward; lateral margins straight or slightly reentrans; umbo narrow, elongate, extending far back over the hinge; beak prominent, pointed, strongly incurved like a hook.

Dorsal valve very shallow, attaining less than one-fourth the thickness of the ventral valve; outline subpentagonal, with the greatest width distinctly in front of the mid-length; moderately or gently convex in lateral profile; medially depressed to form a broad shallow sulcus in the anterior region of the valve; umbo widely swollen; beak small, concealed.

Measurements (mm)

| Length | Width | Thickness Hinge-width Mature specimen 8008 63.6 40.9 34.3 13.5 Immature specimen 8010 31.6 21.6 18.1 12.2

Holotype, I.P.A.S. 8008, paratypes, I.P.A.S. 8009, 8010.

Horizon and locality: Lojoping Village, Ichang District, Hupeh Province. <

Collectors: Y.S. Chi, T. Y. Hsu and the author.

Discussion: This species is abundantly distributed in the Middle Silurian Lojoping Series of the Gorge District, west Hupeh Province. It has originaily been identified with the European species P. borealis Eichwald by Fresh and Hayasaka. The differences between these two species are apparent. In the Lojoping specimens, most of the mature shells are quite large, ranging from 60 to 65 millimetres in length, therefore, it attains over one-third the size of the typical Pentamerus borealis. Pentamerus dorsoplanus difters from P. borealis further in having an exceedingly shallower and less convex dorsat valve, and a pronounced and broadly depressed sulcus. In dorsal view, its general outline is subpentagonal rather than elongate-ovate, and the maximum