Stenothecoid molluscs from China 
213 
anticlockwise during growth, strongly protruding 
and overhanging adapical margin. A prominently 
thick ridge starting from apical area, extending and 
widening to the abapical margin. Ventral valve 
smaller and lower than the dorsal one. Apex large, 
strongly rounded. 
Description 
Shell very tiny, 1.1 mm long, 0.45 mm high and 
0.5 mm wide; bivalved, asymmetrically 
inequivalved and narrowly elongated, about one- 
third as wide as long. Two valves closed very tight, 
commissure of valves simple, commissural line 
curving slightly downward, in particular anteriorly 
and posteriorly. Conjoined line not known in 
detail. Apical area strongly arched. Dorsal valve 
high, strongly arched, the apex being at about one- 
third of the length. Apical area high, strongly 
protruding and overhanging the adapical margin 
and curving anticlockwise during growth. A 
prominently thick ridge starting from the apical 
area, extending backward for about two-thirds the 
total length and then abruptly expanding and 
widening to the abapical margin. The ridge is 
bordered by two narrow furrows, occupying about 
one-third the width at the middle part of the dorsal 
valve and then almost the whole part at the end. 
The lateral slopes slightly depressed and then 
oblique to the lateral margins. 
Ventral valve smaller and lower than the dorsal 
valve. Apex large, strongly rounded and convex, 
gradually sloping toward the abapical margin. The 
lateral slopes of the apex abruptly convex and then 
flatten to the margin. 
The surface character is unknown, except for 
some granules in the dorsal valve. 
Remarks 
Judging from the general morphological 
characters of the shell, Stenothecoides yochelsoni sp. 
nov. is similar to S. biighti Yochelson (1969: 59-60, 
figure 2) from the Early Cambrian of Yukon, 
Canada. They share such common features as: the 
asymmetrically bivalve shell, the possession of a 
prominent ridge on the dorsal valve; and simple 
commissure of the valves. However, there are still 
great differences between S. yochelsoni and S. 
knighti, essentially in the shell being narrowly 
elongated in dorsal valve view in the former, but 
subelliptical in dorsal valve view in the latter; in S. 
yochelsoni the commissural line is curving slightly 
downward, whereas in S. biighti the commissural 
line is curving slightly upward; in the dorsal valve 
of S. yochelsoni the apical area is strongly 
protruding and overhanging the adapical margin, 
but in S. biighti, the apical area is low, slightly 
overhanging the ventral valve; in the former, the 
dorsal valve is strongly arched and has a 
prominently thick ridge beginning from the apical 
area, extending backward for about two-thirds the 
total length and then abruptly expanding and 
widening to the abapical margin, while in the 
latter, the dorsal valve is low and has a narrow 
ridge; in S. yochelsoni the ventral valve is small and 
low, with a large and rounded apical area, but in S. 
knighti, the ventral valve is relatively convex and 
with a curved ridge. 
In possessing a narrowly elongate valve, S. 
yochelsoni is closely related to Stenothecoides sp. 
(Yochelson 1969: 51, figure 1) from the upper 
Lower Cambrian of Siberia, but it is strongly 
distinguished from Siberian species in the more 
convex dorsal valve, the less convex ventral valve, 
in the presence of a distinctly thick ridge on the 
dorsal valve and the more protruded apical area. 
In some respects this species resembles 
Stenothecoides bellus Koneva (1979: 26, pi. V, fig. 1) 
from the Early Cambrian Lenian Stage of central 
Kazakhstan. It differs from the latter in the more 
narrowly elongated shell; the thicker ridge, the 
more protruded apical area and in the smaller 
ventral valve. 
Etymology 
The specific name is in honour of Dr Ellis L. 
Yochelson of the Department of Paleobiology, 
National Museum of Natural History, Washington 
D.C., U.S.A. 
Occurrence 
Upper part of the Huangshandong Member of 
the Early Cambrian Tongying Formation at 
Tianzhushan of Liantuo, Yichang, Western Hubei. 
Stenothecoides sp. 
(Figure 2 H-I) 
Material 
A single specimen, NIGP 116391, in the 
collections of the Nanjing Institute of Geology and 
Palaeontology, Academia Sinica. Collected by 
Zhang Sen-gui in 1978 from the base of the Early 
Cambrian Xidashan Formation on the northern 
slope of the Mohurshan of the Kuruktag 
Mountains, Xinjiang, China. 
Description 
Shell of large size, 13 mm long and 6 mm wide; 
asymmetric, fairly elongately ovate in plan view. 
Apex sharply pointed, distinctly protruding and 
overhanging the adapical margin and curving 
anticlockwise during growth. In plan view the 
valve is convex and tends to be most pointed at the 
adapical margin; narrowly rounded abapical 
margin and somewhat bent to right. Sides 
unequally convex, with the greatest width of the 
valve at about one-third of the length. 
