Stenothecoid molluscs from China 
211 
Mollusca. The fauna consists of the genera 
Yangtzechiton and Luyanhaochiton (Polyplacophora); 
Yangtzemerisma and Merismoconcha 
(Merismoconchia); Eosoconus and Archaeotremaria 
(Monoplacophora); Yangtzespira and Archaeospira 
(Gastropoda); Yangtzedonla (Bivalvia) and 
Heraultipegma (Rostroconchia) (Yu 1979, 1984 a, b, 
1985, 1987, 1990; Jiang in Luo et al., 1982; He and 
Yang 1982). The new species of Stenothecoides 
described herein, like other Meishucunian 
micromolluscs, is characterized by its tiny shell. In 
general, the usual size of Stenothecoides is 4-10 mm 
in length, though specimens can reach a length of 
14 mm or so. However, the Meishucunian species 
is 1.1 mm long. 
The discovery of Stenothecoides from the 
Meishucunian Stage provides important material 
evidence for study of the origin of Stenothecoida 
and the evolutionary relationship with other 
molluscs. Of special interest is that most Early 
Cambrian species of Stenothecoides possessed a 
distinct ridge or fold on the dorsal valve or right 
valve of variable expression (Rasetti 1954; 
Yochelson 1969; Koneva 1979; Peel 1988), while in 
the Middle Cambrian, only a few species have a 
ridge on the valve. The new species of 
Stenothecoides has a thick ridge on the dorsal valve, 
occupying more than one-third the width of the 
valve. These characteristics indicate that this 
species is a primitive species of the genus 
Stenothecoides. 
It is worth noting that the upper part of the 
Huangshandong Member bears a rich 
micromolluscan fauna. The polyplacophorans, 
monoplacophorans, gastropods and rostroconchs 
either lived a benthonic, creeping life within the 
subtidal and intertidal zone or were infaunal 
deposit feeders. Most shells are preserved 
incompletely, some overlap with each other, while 
some are fragmentary. These characters show that 
they were deposited in the turbulent flow of a 
shallow sea or even in the littoral zone. 
One of the forms of stenothecoids in the 
Xidashan Formation at Kuruktag, Xinjiang is 
Bagenovia cf. sajanica. Horny in 1957 named the 
genus taking Bagenovia sajanica Horny as the type 
species. Bagenovia sajanica was first reported from 
the Early Cambrian of west Sayan, Siberia. This 
genus has been found in the Early Cambrian of 
Siberia (Radugin 1937; Horny 1957, Sytchev 1960; 
Aksarina and Pelmen 1978) and the Early 
Cambrian Lenian Stage of central Kazakhstan 
(Koneva 1976, 1979). In China, it has been found in 
the Early Cambrian Xidashan Formation of 
Kuruktag, Xinjiang, the Early Cambrian 
Wuxinzhen Formation of Yichun, Heilongjiang and 
the Early Cambrian Shipai Formation of Suizhou, 
Hubei. Two specimens were procured from the 
Xidashan Formation, including one covered with 
radiating costae and one well-preserved internal 
mould with internal structures. 
The interna] structures of the valve consists of 
four pairs of asymmetrically arranged ridge-like 
impressions and three concentric impressions 
visible on the dorsal valve of the internal mould 
(Figure 3). The structure may correspond to 
anatomical features, but the soft tissues of 
stenothecoids are quite unknown. Since they have 
no expression of the outer surface and the direction 
of the ridge-like impressions are the reverse of the 
radiating costae, it seems likely that they reflect the 
impressions of some organs of the animal. The 
function of the internal structures is, however, 
difficult to interpret. It might be assumed that they 
either had a direct effect on their free movement 
or, alternatively, relate to the control of the opening 
and closing of the valves or some other 
physiological action, perhaps representing muscle 
attachments (Yochelson 1969). 
The taxa Bagenovia cf. sajanica and Stenothecoides 
sp. co-occur with shallow and warm water fossils, 
such as archaeocyathids and the trilobite 
Metaredlichioides and Chengkouia. Most specimens 
of archaeocyathids, trilobites, brachiopods and 
molluscs are preserved incompletely, some of them 
disorderly preserved in the limestone, others as 
fragments (Zhang 1983). These characteristics 
indicate that the fauna reflects a shallow water 
carbonate facies, perhaps deposited in a 
epicontinental sea. Their associated sediments, 
arenaceous limestone, are characteristic of shallow 
water deposition. 
SYSTEMATIC PALAEONTOLOGY 
Class Stenothecoida Yochelson, 1968 
Superfamily Cambridiacea Homy, 1957 
Family Cambridiidae Homy, 1957 
Genus Stenothecoides Resser, 1938 
Stenothecoides yochelsoni sp. nov. 
(Figure 2 A-F) 
Holotype 
NIGP 116387, in the collections of the Nanjing 
Institute of Geology and Palaeontology, Academia 
Sinica, collected by the writer in 1978 from 
Huangshandong Member of the Early Cambrian 
Tongying Formation at Tianzhushan of Yichang, 
Hubei, China. 
Diagnosis 
Tiny, bivalved, asymmetrically inequivalved, 
narrowly elongated, about one-third as wide as 
long. Two valves closed very tight, commissural 
line curving slightly downward. Conjoined line not 
known in detail. Apical area high, curves 
