Scientia Sinica

174 SCIENTIA SINICA Vol. V

having a width greater than length, greatest width about near the mid-length; anterior margin truncated; lateral margins broadly rounded; lateral profile very unequally biconvex; anterior commissure strongly uniplicate.

Ventral valve flatly convex; postero-lateral margins slightly depressed; median portion moderately elevated; umbonal region narrow, highly arched; beak small, slightly incurved; sulcus very broad, shallow, equal to more than half the width of the valve; tongue exceedingly long, bottom flat, bending almost in right angle or a little recumbent posteriorly.

Dorsal valve strongly convex and most convex in the anterior margin; posterior and lateral slopes curving abruptly and steeply toward the plane of the commissure; fold low, broad, defined only in the anterior half of the valve; umbo widely swollen; beak concealed under the ventral beak.

Surface marked by broad flat plications, separated by much narrow and shallow furrows; number of plication 7 in sulcus, 6 on fold. and about 11 or more on each lateral slope.

Dimensions (mm): Length 18.3; width 20.4; thickness 15.2. Holotype, 1-P.A.S. 8017.

Horizon and locality: Upper Devonian; one mile west of Yenmuchiao, Lingling District, Hunan Province.

Collector: K. K. Chao.

Discussion: This species is represented by a single well preserved specimen and is the first known species of Hypothyridina from the Upper Devonian in China.

This species resembles Hypothyridina cuboides (Sowerby) "*! in its general shape and outline, but differs in being comparatively smaller in size, in having more surface plications and a much longer sulcus. It resembles also Hypothyridina emmonsi (Hall and Whitheld) (51 from the Independence Shale of North America in proportion and outline but differs from it in the number of plication both in sulcus and lateral slopes. As described by Strainbrook, in mature specimen of H. emmonsi, there are 14 plications in sulcus, 15 on fold and 18 or more on each lateral slope.

Genus Wellerella Dunbar and Condra 1932 Wellerella subdekalbensis Wang (sp. nov.) (Pl. V, A; Figs. 15)

Shell substance very thin; size small; outline subtriangular to subpentagonal, having a considerably greater width than length and a bit greater length than thickness, greatest width near anterior margin; antero-lateral angles broadly rounded; very ‘inequivalved; anterior margin truncated ; anterior commissure widely uniplicate.